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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