The problem is coming up with challenges that are not trivial to automate and not annoyingly hard for users.

Botters aren't just making passive income, they can fix their bot every day as long as it's making them money. Many sites try "random" questions which are really easy to pattern match with a little regex.

The amount of effort that goes into botting generally scales linearly with userbase size (not always). But once you're talking about a botter with a profit motive when your site has hundreds of thousands or millions of MAU, I think trying to come up with challenges to beat botters would be prohibitively time consuming.