It's the same with Github, sure you can host on your own domain, but "search for voctoweb on Github" is so much easier than "oh just go to my homepage at sdkljfsdl.sdf".
If you use google instead of github search you can find it even if it's on a self-hosted gitlab, though. And since both use git as API nobody is stopped from forking from gitlab to github either.
Yeah good luck searching for something unfortunately named that doesn't already happen to be super popular, though.