Go with the flow, don't rock the boat and use what developers already know, are probably the most cited reasons I've heard.

I've tried so many times in the past to argue for self-hosted setup that you fully control if you can afford it, things just get so much smoother and if you're a software development company, you probably want to own the software development workflow E2E so you can actually ship as fast as you want.

Usually, at large enough corporations, it's one of two things. Some random project gets open sourced, and it ends up on Github(see, for example, Salesforce) - or, more commonly, some subsidiary or acquisition had github and has either refused to migrate to the internal source system or the hassle of migration isn't worth it.