Dealing with github is the boring and tedious thing, you have to run huge amount of proprietary javascript, keep up with their weird UX changes, start X11 to open a browser to render their html, overclock your CPU for a large PR review conversation to scroll without locking up your computer for minutes, constantly click "load more" since their webpage keeps hiding comments (while still lagging massively)...
Email is simple. It's just text, there's no weird javascript or html or lag. I don't have to open X11. I can just open mutt and read or write. I can type "git send-email". It's all open source, so I can read the code to understand it, and write scripting around it. It runs on any computer with ease. Even on a slow connection, it's quite speedy.
I totally agree with you about Phabricator though.