Is there some reason why voting bots can't just be banned? Is it logistically impossible?

There is no reason why someone with a considerable amount of SP can't spend an hour a day curating, considering the rewards at stake. If you want to set it and forget it you can still delegate your SP to a manual curator.

In terms of the pay for upvote bots I think one big factor is that the promoted section seems to be fairly ineffective. I have used it a few times with dismal results. It looks like you would have to spend at least 20-30 SBD to get noticed which is hard to scrape up if you are a minnow. If you have a quality post and throw a few dollars into a black hole you should at least get noticed by a few people. This of course wouldn't solve the problem completely but many people use paid for bots to get their post on the trending and hot pages to get more views and followers not because it can be immediately profitable.