How you do PRs is definitely important, and should be part of a company's consideration here, but remember that PRs are already a layer of abstraction over the SCM. Self-hosting Git is definitely not as easy as setting up a GH account.

Not to mention that you get an easy way to spin up your CI/CD workflows in GH Actions (which of course definitely has its own problems, which there was another popular HN post about recently). There's a reason why it's the default for new companies -- if there was something much better, it wouldn't have the market share it does I think. Familiarity coming from OSS is also important.

Most frequently, I think it's because you want a single platform to store all company code, but not all teams agree on using GitHub PRs vs a more advanced code review tool. Other potential reasons include having access to the other GitHub features: issues, actions, security stuff, the merge queue, etc. You could pull all these together from less-overpriced more-specialized alternatives, of course, but sometimes it's nice to have a single integrated platform even if you decide to replace one of its features.

GitHub is a platform with dozens of tools, I think of it as a basic toolbox. It's great, but if you're hammering nails all day, you should invest in a nailgun. Doesn't mean you don't use the hammer or the wrench in your toolbox, but when you care about one task a lot, you invest in the tool to do that task better