If some dev wants to make a monetized platform that uses activitypub, they can, and they can do it literally however they want whenever they want. I’m sure threads is or will be monetized in some way.

Literally nothing is stopping anyone from doing that.

But most current users probably wouldn’t migrate to a monetized platform, or even really want to interact with one, and new ones who do want to monetize probably won’t even move until a critical mass of people moves from the old platform(s). So there’s no incentive to create that at this point. And when the critical mass who wants to make money moves, they can figure out how they want to monetize their own platform.