Good question Hive account@jorlauski. Let me give some further explanation on this one.

They don't need to read all the posts/comments to find the best ones. On the first step a simple bot filters off 99% of posts/comments (it is really not a difficult bot based on criteria: for example, it is clear that bot will need to filter short comments like "good post!" or posts having a couple of "f*ck" inside, or posts/comments with lots of misspellings, etc. - doubtfully look like a TOP-30 candidate).

Then humans manually read the remaining 1%. To be even more fair, human curators will also look through all the top posts/comments already chosen by the community (based on # of votes & comments).

Award 30 out of thousands? Yes, why not? Quality is the key. Share deep life-changing thoughts and ideas! If you really do your hard work every day, then one day you will get your $250 upvote, and after such an upvote you will get your Reputation boost, followers, money to "Promote" your further posts... Don't you think it is a good motivation to spend your time on quality content? In fact it is almost impossible to get even a $10 upvote from the wales today (until you are a whale too) - but this stops thousands of new users to dedicate their time to Steemit!