Why would you think so? Can you give examples of specific tools that wouldn't be available to mail clients? On the other hand, there are many things available on most email clients which are missing on GitHub, like tagging automation from custom and flexible rules, Turing-complete filtering, instant searching, saved searches, managing the lifecycle of issues, linking with the VCS etc. all in context and in one place.
How people generally go about re-implementing those on GitHub is with bots, and you are left at the mercy of what the bot can do/its admin wants you to do, and each project is its own silo and possibly breaks your workflow.

I'm fine with GitHub because these days I'm mostly a casual contributor, but there's a lost appreciation for the sheer power and universality of email-based workflows. That the largest projects (including the Linux kernel) run on that should speak for itself.