Do they? Unless you're on GitHub Enterprise, migrating is just moving your repos over the weekend, setting up new webhooks, emailing everyone a command to switch their upstream URL, and hoping the new workflow works for you. For teams of <100, this it one of the easier transitions to make.
[1]: https://docs.gitlab.com/ee/user/project/import/github.html
[2]: https://confluence.atlassian.com/get-started-with-bitbucket/...
Update: apperantly github also has self hosted runners
https://help.github.com/en/actions/hosting-your-own-runners/...