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 :-)
For early development that makes sense. But if Gitea is not ready to self-host now then it is also not ready to host most other projects so that doesn't really invalidate the criticism. However from the sibling comments it seems that the move to self-hosting is in progress.
It looks like the only feature blocking their move to self-host is importing data (issues and PRs) exported from GitHub. Which is to say, it's perfectly ready for hosting new projects.
Sibling has already provided the tracking issue for getting off it; as far as I remember it's close now :-)