There are few reasons that people do so. The biggest reason is some of the whales like Jerry Banfield telling people that steemit is amazing by just clicking upvote and get money.

In one of his video he keeps clicking upvote and telling people "look it is just crazy" with his face coming out of the screen.

He also tells people "aha, look. you just click on your comment and you get paid for it? Man, it is crazy. Steemit is different".

People watching videos like that exactly follow the instructions without creating real value. This adds to a huge spam on steemit. The result is those who are here for a quick buck do snatch it, but those who are innocent lose their voting power and abandon steemit despondently.