The nearly 4B market cap public company I work at uses gitlab to host our internal repositories. It works really well for us, we get a lot of the benefits of github, but we have control.

Huh.

Okay, I guess that makes as much sense as anything really. I can see the advantage in something turnkey.

Large organizations do it for different reasons. To use their own directory service (LDAP/AD), to more easily integrate it with their other tooling, to be in control of where their data is, to have more control over availability, to be able to add layers of security (VPN, etc.).