Each season there will be many Light Novels getting adapted into an anime. Each season, the same story will be told again with a different skin. Each season, the same "generic" protagonist will be put into a new world, with little differences, and go to a journey that could be described: "same as X but with Y."
Those who watch seasonal anime for a long time might have noticed that Light Novel protagonists seem to be similar to each other. So much that if you swapped some of them, their stories might still stay the same. (According to the examples I've seen though, this isn't as common as anime fans will tell you.)
This article's examples will only be Isekai and similar types of the Light Novels. The problem is there for all Light Novels (even Manga) that can be traced to a certain template even if it's not the Isekai template.
The problem with Light Novel Protagonists is that while all of them are different, most of them are not interesting to watch, especially compared to the usual Manga Protagonists and especially when animated. I said they are not interesting to watch, but most of them are interesting to read! Light Novel protagonists work completely fine in Novel format, less so in Manga format and it's completely hard to do them right in animation or live-action at least without changing the story. (The Anime creators wouldn't want to change the story because Manga/Light Novel fans will rage if that happened. They are the target audience after all.)
The Importance of First-person Perspective
Two of my favorite Light Novels: Sword Art Online and A Certain Magical Index had protagonists that are very interesting to read in the light novel format. Usually because of how the story is written, but also because both Kirito and Kamijou Touma respectively, have a type of inner conflict in them. The reader has access to their thoughts at all times, and their thoughts are interesting to read, but they wouldn't be interesting in anime format. They will drag the story on, make the anime longer and screw up the pacing! Even with that, the audience might find them boring, because anime doesn't work that way.
Even Light Novels that aren't written in a first person perspective dive in the mind of the characters. Motivations and emotions are explored more than the usual Manga story does... The style of writing also plays a huge role on making a protagonist interesting. Something unique to text-based works, is that character designs aren't as important as the thought process of these characters, which is why many of the LN characters have similar character designs but each protagonist has a different personality.
Case Study: Kirito and Kamijou
I think many SAO fans, enjoyed Kirito in the Alicization arc of the anime, even if they hated how generic he was in the arcs before. Kirito in the novels was always like that. The anime (is justified when it) didn't adapt Kirito's inner conflict in the arcs before because it didn't affect his actions as much as they did in Alicization to matter to the animators. Again, that inner conflict is interesting to read.
In the anime version of SAO, Kirito's personality is inconsistent, but every thing he does is explained in the novels. The change of personality between different time periods makes sense in the Light Novels because you know where the side story you're reading fits in the narrative. But when you watch the stories in chronological order in the anime, Kirito feels like different character between episodes. Because without the context of the main story, and the thought process of Kirito, the change in personality only makes sense in retrospect.
I have to add here that it's one of the reasons why SAO: Abridged is so popular. This fan-made version of Kirito had a different personality (a jerk who hates humanity) that develops in every episode and every encounter he has in a clear way. You know why he acts different the next episode because you know how he changed in the episode before. He is a better person by the end, (well, only less of a jerk.) Abridged Kirito seems better written because he was written for a visual medium from the start!
The problem is worse with Kamijou (of Index.) He is not the only focus of the story, (the world building gets way more focus than him.) He only gets character development later in the story: His character development starts late in the third season of the anime, and continues for more than half of New Testament Light Novels. But even before his character development, the way the Light Novel is written, Kamijou is one of my favorite protagonists of all time. (He is surpassed by Accelerator from the same novel though.)
All villains in Index/Railgun universe, have legitimate reasons for their villainy and when you know about them as a reader you empathize with them... Kamijou will always provide a better solution to their problem! A solution that makes their evil plan or at least the sacrifices of it unnecessary. You see all those villains who are powerful and smart enough to destroy the world, and the powerless Kamijou will sound Better than them in the end.
It's hard to not respect a character like that, but the anime only shows the conclusions Kamijou reaches, no the thought process. It didn't do it justice. It can not in an easy way, and definitely not by adapting each Novel Volume in 4 or less episodes!
The Target Audience
One interesting thing about Anime adapted from Light Novels is that the goal is not to make a masterpiece anime. The goal is not to change the opinion of those who got tired of the same concepts being rehashed in different ways. The goal is to sell the Light Novel itself!
When a studio makes an anime based on a Light Novel. The audience that they make the anime for is the audience that is already in love with most of Light Novel tropes. They don't have to hold back on the harem elements because their goal is to sell all of these characters to the fans who prefer one "love interest" over another! (This is bad for the anime itself, but the anime wouldn't exist without them.)
The target audience is also the reason why tsundere characters have their tsundere-ness getting worse in the anime version, while the Light Novel version is "less annoying." The audience who throw money at their favorite tsundere demand such things. (Shana/Louise/Taiga/etc are more similar in the anime than in LN versions.)
The target audience for anime based on Light Novels are different than the general (or Manga-based) anime fans and unique enough that it's not hard to imagine why there are many anime based on Light Novels:
Light Novels fans buy... A LOT! Light Novels are cheap and their audience is massive. The audience of one Light Novel will love to read other novels that touches on the same concepts. Since they're all written differently, you'll see more differences between two Isekai Light Novels, than the anime adaptions of both of them.
Anime only serves as a Promotion/Advertisement instead of a standalone show! You can watch Naruto/Dragon Ball/Death Note/Attack on Titan as anime only and still get a very satisfying experience... Anime based on Light Novels expect the audience to buy/read the Light Novel version. They leave some characterization out. Leaving only the bare-minimum for the story to progress, saves time/budget and leads to adapting more arcs per season. After all the target audience already know that stuff.
Light Novels with anime adaptions just sell better! So, you've got an audience that BUY A LOT, but there are more Light Novels than a person or ten can buy. (LNs are very easy to write after all.) So when the publisher sees a potential of one of their works, they sponsor the anime version of it and see how it would go! The sales usually skyrocket as soon as the anime is announced!
So, you get a huge portion of fans who will only buy a Light Novel if it got popular enough to get an anime, but after the first season the anime becomes less of necessity to the publisher because they already accomplished their goal... That's why Light Novel anime is rarely finished! They usually get an open ending and the only way to continue the story is to buy more Light Novels.
What do you think?
So, this was my take on anime based on Light Novels... They are not bad. Only similar to each other (generic to their own genre.) The reasons for that in my opinion is a combination of "how the original Light Novels work" on top of "the goals for the publisher/studio making said anime." Business stuff makes the anime worse than it should be, but without the business side it wouldn't exist in the first place!
After all is said, I'm happy that we at least have this compromise. While many of those shows are "generic" the audience loves them, and we sometimes get something that blows the fandom away like ReZero. Finally without all of these "generic" shows, we wouldn't get stuff like Ascendance of a Bookworm animated: It's too risky but was worth it in the end!
I hope you learned something from all of this. This article was surprisingly easy to write. I already researched this before and this was just arranging my thoughts... Please tell me if you disagree with any point I said because it's just a personal opinion.