To be fair, there wasn't that much sensitive web content around in the 80s to leak (primarily due to the web not yet existing, nor browsers), so it's only fair that browsers didn't consider using IPC for site isolation back then.
Firefox has shared a lot about their efforts in moving there. If you're curious, there are a lot of blog posts and internal design docs under their project name "Project Fission".
But yeah, the fact that both Chrome and Firefox have managed to do so does leave Apple looking slightly bad here.
Performantly implementing that API across processes is possible, but not quite trivial.