Why Linux and Git development happens on mailing list instead of GitLab for example.

Git was specifically CREATED to facilitate this exact mailing list workflow.

With git you don't have to push and pull to a repo, it also works by sending signed commits over any method you want.

Linux development includes a large number of contributors who use email. Git was originally created specifically for Linux development so of course it caters to this approach.

There's also the fact that the lead devs for Linux serve as a clearing-house for patches, so receiving them over email with explanations and everything helps them decide what makes it in.

Pull requests are not the only way of doing things. Generally speaking git can do a lot of workflows and architectures but most people are only familiar with gitflow and PRs.

I was under the impression it was primarily " if it isn't broken, then don't fix it" levels of tradition.

I could be wrong though, so if someone with more creds than a random internet dweller comes along, feel free to ignore me.

Heck, if you think I'm an idiot, feel free to ignore me :)

Probably because it works and there hasn't been a strong reason to change it.

It's obvious in the case of git, git development occurred before gitlab existed.