How would that work? Besides starting completely new centralized git service to compete with github and co? Anyway, git already has a "pull request" system in the form of git send-email and related commands.
I know Linus probably would not agree but emails aren't a substitute for a pull request system: they're a bandaid to compensate for the lack of one.
I agree it would require additions to git & some kinda of standard around it but it's telling that every major git system in use "solves" this problem.