>but what I'd genuinely like to know is why don't big projects just move to their own thing?
Github is a great advertising and marketing platform for large OSS projects. Quite the opposite, large projects should be moving towards github, because it's a great stage for them to perform on. On top of that, it's rather become the defacto replacement for sourceforge (sorry FOSShub, it was a bold try), with all that implies.
The thing I really don't get is why people use it for small projects. Like Facebook, Twitter and basically every other social media site out there, it's a drama factory. It incentivises people towards public grandstanding on creative and technical issues, and encourages resolving disputes by forking the repo, burning bridges and splitting the dev team rather than discussing all aspects of the problem, chewing it over and making a group decision.
That's the last thing small OSS projects need, and I'd have thought most 5-15 man projects would be vastly better off throwing up a kallithea repo (if they don't just use DCVS the real way) and a dokuwiki and then knuckling down to business. I acknowledge that this reduces discoverability by some amount and thus risks taking a hit to your developer acquisition, but I'd love to see some hard numbers on how much. I'd wager that much like the Apple app store, if you're not in the top 50 on github, you're no one.
But hey, I'm apparently an old fogey born young, and I prefer to self-host when I can, so maybe I'm just pointlessly resisting the tide of the inevitable or something.