Auto-tagging is not a business they want to be in, because they will invariably get it wrong sometimes and people will start drama over it. And they'd be right to: human communication is nuanced and you can't just "guess" what someone means within 280 characters.

People already use hashtags, what would happen if you could subscribe to them? It would create a whole new way of interacting with the platform, something closer to Reddit where people post to streams. But the fact that these streams can't be moderated (or even downvoted) might be a nonstarter.