I'm not sure, but my understanding is that GitHub historically focused more on open source, with PRs being mainly across repos of unrelated users, such that there's more of a distance to "pull" across, while Gitlab was always mainly targeting companies, where people typically use branches in the same repo, so it's just a nearby merge.

In other words, I see a pull request in an open source project to be just "I have something nice in my fork, do you think it'll be useful upstream?", which is acceptable to reject, whereas in a team setting it's "I have a feature that I think is ready to merge - give it a look and see if I missed something before we put it in".

This is really helpful framing. Thanks!