> Honest question, why are companies interested in hosting on github?

Mostly boils down to marketing and easier to establish a community. Almost every developer has an account there, leading to network effects being much larger, so if you're a new FOSS project, finding contributors and getting your project in front of other's eyes is much easier when you're on GitHub compared to your own Forgejo instance.

With that said, I'd question if chasing "most external one-time contributors" or GitHub stars is the right way to actually run a FOSS project, personally I'd avoid thinking about those vanity-numbers as much as possible and focus on the project, code and contributors themselves.

But, I've literally heard those two arguments for "why GitHub" countless of times over the years.