What if the one of the authors themselves do it? And it is also meant to be free according to them?

I am the author. They are Free.

Yes, I got that. My question was- what prevents you from simply uploading a PDF of the book to GitHub, GitLab, etc.?

I used to use Sourceforge. They became unacceptable. I switched to Google code. They turned off, as I understand it because people were uploading pdf's (pitated ones, but big tech seems not to want to make a distinction like that). Then I used GitHub and in the run up to getting bought they made some changes that I thought looked ominous. So I am now at GitLab. Can you blame me for wondering if it is too much to hope that a site will freely host me for, say, a decade even if I go away?

By the way, I put a stable version of my latest on GitLab.