Because you are cheating, plain and simple. You earn an audience you don't buy it, but if you have the money we have a sensible function to promote, it's under all active posts called aptly the Promote Button, and all the money goes to get burned thus increasing the value of SBD and in turn Steem while you aren't effectively buying your reputation and curation and only benefit in the exposure of being promoted, thus not invalidating the function of curation or rewards in turn, or the basic principle of earning your audience. If you use vote bots you're paying for what people ought to give for free, or their vote.