"I think in the end we should not expect GitHub to provide the best option here. We should expect them to provide a basic option (which they do) and for sophisticated consumers to pay more for a much better option. Everyone should be shopping for code review tools!
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I understand this linke of thinking might suit you but I fear it is not as convincing as it sounds to you. At least it's not to me.
Here's how I like to think about it: GitHub is a generalist. They have a big platform with lots of features besides code review, so even though they also have lots of employees they won't be able to focus on code review as much as a dedicated company could. They also have a huge number of users to please so they can't afford to rock the boat too much or make the learning curve too steep.
I think therefore it's pretty much inevitable that if you need a more advanced code review tool you'll end up picking a third party one. Though admittedly, as the founder of Reviewable, that thinking does rather suit me too. ("It is difficult to get a man to understand something when his salary depends on his not understanding it" and all that. :D )
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
I understand this linke of thinking might suit you but I fear it is not as convincing as it sounds to you. At least it's not to me.