GitHub is a much larger target, and has many, many more moving parts than your average Gitlab on, say, a Linode virtual server.

Github will probably have more downtime which you can do noting about, than your selfhosted gitlab, which will probably fail because you upgraded something, or tinkered with a configuration.

It's a numbers game. They have more people monitoring, they spend way more money than you do on uptime/services, but such a service doesn't get more stable as it grows.

I trust myself way more than I trust GitHub or other hosted/cloud services because I cannot affect/help them in any way when stuff happens.

Ultimately, I know this, I accept this, and I happily use GitHub, Bitbucket and such all the time.