That's actually an argument in favor of git.
SQLite has literally <= 3 developers.
SQLite has had about 33 contributors (after you combine people using multiple login names). But 92% of the commits have been from just two people. 97% from just three people. Then there is a long tail of other contributors.
Fossil itself is similar: https://fossil-scm.org/home/reports?type=ci&view=byuser - 5 or 10 people account for most of the commits, and then there is a long tail.
But aren't most projects like this? A few core developers are responsible for most changes and enhancements, and then there lots of others that might contribute a patch or two here and there? I suspect there exceptions to this (the Linux kernel comes to mind) but I think they are rare. Correct me if I'm wrong.
If we accept drive-by patches from anonymous users, then SQLite would have to be relicensed as GPL or similar, which would undermine many of its use cases.
In the US, authors cannot put their works in the public domain. This ability doesn't exist in statute, nor have the courts accepted the concept. Past attempts to allow this have failed in Congress. One can refuse to enforce their rights on creative works, but copyright still exists from the moment of creation and works won't enter public domain until the expiration of the copyright.
Furthermore, the US allows authors (except work-for-hire) and their heirs the right of termination for copyright transfers and licenses. This is an inalienable statutory right which cannot be restricted by contracts.