While this is true, there's more history. At the time #ActivityPub became recommendation the people involved knew that important improvements were required in future iterations. This just never happened. Specs never evolved and projects created their own solutions.
For many years, what's now #Streams and #Nomad protocol, created by @mike has had nomadic identity. Why this hasn't picked up by other projects? Idk, getting such adoption is likely just as hard as evolving specs.