Pull request is common feature for maintainers in the Linux world, and they don't use github to send them to Linus.
And on top of that they are secure (signed tags) and decentralized (maintainer can publish them anywhere)
It's an annoying problem because you need decentralized identity. Signatures handle that but the UX is not as nice as the centralized github/gitlab account.
The design of decentralized PR hosting also doesn't play nice with the github/lab UX. If you were trying to create a PR to a github repo you'd have to handle authenticating the github pull from where-ever you hosted. If the repo isn't public that seems very cumbersome even if it was supported.
It's just much easier to do this entirely inside github/lab, annoying as that is.
Seems like the specific problem is pull requests are not a core git feature in that they cannot be conveniently accomplished in a secure and decentralized way.