Usually complaints like this have more to do with the social processes around coding than the actual task of storing and versioning source code (which as you say is portable and standard).
"I want to make a change to a shared library. Why can't I make a pull request?" "Wait, I have to use this unfamiliar interface to make comments on other people's changes and I can't leave comments on specific lines?" "You know, if you used Jenkins and Github then you could show the status of passing or failing tests right here on the code review screen..."
These social pressures are really quite strong. They affect a bunch of open source projects especially: people who want to make changes expect code to be on Github and might even mirror it there themselves (creating confusing situations for anyone trying to contribute). Even if the project does host its code on Github to allow for contributions from Github users, Github is (naturally) not very good about directing its users off of its platform to where the existing discussion and development is going on. "It's easier if you just do everything on Github" says Github, and their users by and large agree, and slowly more and more process (code review, merging patches, CI, documentation) gets sucked onto Github by the platform effect.