Ideally , Firefox would be downloaded from official Mozilla Flatpak repo, Blender from Blender Flatpak repo on Blender server, etc... And we would have this list of those repo on our distro.

However because the official adoption is slow (very few software have official flatpak repo), flathub allowed the community to build packages themselves. But this is clearly not how it should be.

The second (more legit) reason of flathub is that small developer might not want to pay a server to host their app and flathub proposes to host their repo.