Technically you can do everything through email, because everything online can be represented as text. Doesn’t mean you should.
PRs also aren’t just a simple back and forth anymore: Tagging, Assignees, inline reviews, CI with checks, progress tracking, and yes, reactions. Sure, you can kinda hack all of that into a mailing list but at that point it’s becoming really clunky and abuses email even more for something it was never meant to handle. Having a purpose-built interface for that is just so much nicer.
That’s kind of weird, because the reason why I never bothered with (selfhosted) VPNs before Wireguard was because it was the first one that just worked. Granted, due to its nature, you don’t get a lot of feedback when things don’t work, but it’s so simple in principle that there’s not a lot that can go wrong. For external VPNs like this, it should just be: Load config, double-check, done.