Another critical reason is that GitHub is proprietary and has a pile of services with lock-in that (unlike git) aren't easy to move elsewhere. Some people depend on GitHub for their livelihood (through GitHub Sponsors). Some people's software integrates tightly with GitHub bots, or Actions, or issues, or project management. Everything other than the code itself is incredibly difficult to port over to another service.
Another critical reason is that GitHub is proprietary and has a pile of services with lock-in that (unlike git) aren't easy to move elsewhere. Some people depend on GitHub for their livelihood (through GitHub Sponsors). Some people's software integrates tightly with GitHub bots, or Actions, or issues, or project management. Everything other than the code itself is incredibly difficult to port over to another service.