[spadufed]

I think what you're describing IS activitypub. The major issue is that each implementor has a preferred message type and only basic functionality of the others. I've posted this elsewhere in the thread, but Lemmy top-level posts are implemented as Pages and communities are implemented as Groups. Currently Mastodon has implemented both but with limited functionality. There is a rework of Groups that is listed as in-progress on the roadmap (https://joinmastodon.org/roadmap).

I think the way we ended up in this situation is that there are quite a few types of messages in the AP protocol, and up until recently the up and coming ones were seeing pretty limited use. Without another service to test against, I can understand why a lot of fediverse developers have opted to kick that can down the road. That said, we are definitely in the era of determining the defacto use case for each of these message abstractions, and I suspect that will be a slow process and involve a lot of back and forth between projects.