While I could theoretically run stuff I want to share off my own server, I'm also realistic in that I know I won't be able to dedicate the time required to make it work. It's never just a server, to do things properly you need fail overs, backups, alternate sites, etc.
And then it's just way too much work required just to share a single Git repository. You're right, I don't like system administration much.
I do run my own server, but I run it for myself. I know the backups are irregular, that there's no alternative site. But if it breaks no-one else is affected.
The Github's of the world fulfill a need, that's why they're there.