(⚠ The following sections are a work-in-progress.)
Simple: too much centralization. It usually doesn't play out very well.
Yes -- and no. You can't sign up on this Gitea instance (I have disabled it) and therefore can't open issues or pull requests (a workflow you might be well oriented with), but you can still contribute using a combination of format-patch and send-email. Patches are built-in to git and how large and established projects like the Linux kernel receive contributions.
In the long term, my hope is that where your code is hosted will become irrelevant.