Yeah, the motivation question is definitely a tricky one. A common REST story also feels like a piece of eventually getting to federated PRs between forges, though it may well be that that's just impossible, particularly given that GitLab has been thinking about it for a decade and hasn't even got a story for federation between instances of itself much less with Github or Bitbucket:

https://gitlab.com/gitlab-org/gitlab/-/issues/14116