I couldn't agree more and I have definitely seen @everittdmickey discuss this before on some similar posts, I believe the problem with implementing this on the blockchain is that Captcha requires a "Centralized Validation", I believe Dan commented about it Here, this problem means that the system would no longer be "Decentralized" as all upvotes and such would have to go through a central system for verification and hence would defeat the purpose of the blockchain.

I believe the way to deal with this would be to have bots with higher SP (I know Ironic) which would detect bots in terms of their behavior (would detect similarities with comments and be able to identify spam-like comments) and to flag their posts, this is absolutely possible and I have seen and experimented with first hand AI which has sufficient enough language recognition to be able to perform this task.