I've made a few OSS contribs that were fork, clone and edit locally (or edit online in one case), push, test, pull request.

That was all, they took ~20 minutes, if that. Adding in "create an account", "figure out the new UI's method of forking/pulling", and "link myself to another random webservice that I'll only use once" is too much friction, I might spend as much time setting up the account as actually making the edit.

Sure, if I'm planning to become a contributor to a large project (like, say python, which isn't on gh), I'll go ahead and make an account on the bugtracker and join the mailing list and learn how the hell mercurial works. That's fine if I expect to make 10+ commits to a project and really delve deep into it. But most projects aren't big. Most projects aren't that interesting. OSS thrives on people fixing the one bug they encounter (seriously, look at the bus number of major projects). Adding friction in the process is bad for that software.