For many it isn’t easy to just up and abandon what they built on GitHub, especially if they have a big community and open issues and PRs. Familiarity also plays a big role, you can’t simply expect to open an account on a different forge and be done, it consumes time to get acquainted with the new stuff. Also GitHub may give access to more resources: For example, you can just use GitHub actions in your repo, private or public; to use the equivalent on Codeberg you have to request access and be approved.

https://docs.codeberg.org/ci/

None of this is a defence of GitHub. But if you want to enact change, you have to understand the reasons why people remain in the status quo.