Why? Because I don’t want to be trapped into another ecosystem that will do a Twitter/Reddit when it gets big enough. Reddit clones are plentiful, but they’re all fragmented in the user base. At least with Lemmy that doesn’t matter, I can use any community from my home instance.
ActivityPub in itself really isn’t that bad, there’s a lot more that goes into the platform as a whole. And it’s also a good choice because it’s a standard protocol that everyone agrees on. Is it perfect? Probably not. But it works good enough, and we now have Lemmy alternatives in the form of Sublinks and K/Mbin. There’s also a decent chunk of toy ActivityPub projects out there as well.
I can still be trapped in a dying ecosystem but that’s true of every site and at least I have the option of taking my data and converting it to another software if I want to.