That's because when Gitea was early in development, it would have been not usable enough to develop itself (the Gitea people have a post somewhere explaining about it; how too-early dogfooding can actually make things worse as you try to implement 'urgent' features in a rushed manner rather than taking the time to do it right).

Sibling has already provided the tracking issue for getting off it; as far as I remember it's close now :-)