It’s not really explained anywhere, but FBX and rest of the formats come with sketchfab rules, which are basically that it can’t be mature or copyrighted, but UE format comes with a whole rule book from UE marketplace.
The idea is that if someone making a game, he can just import it into his project and it should be usable right away. So it has to have proper textures, collision, sometimes rigged.
I agree that’s it weird that some formats have zero quality control, but UE assets get every detail checked by a real person.