Because they are bad for the system.
Upvote bots allow for reward pool rape. Allow for people to gain power to downvote other people to oblivion. A lot of them are ripoffs.
Bots are what is killing Steemit. And, while I know I can use them to make more votes, I would be a hypocritical to do so.
The more bots are used for automatic or paid upvoting, the more we'll see of crap posts.
Never really thought about it like that