It's a good question - docker doesn't require a VM on Linux, but Docker Desktop does. I assume it's to make it basically the same experience as on Docker Desktop on Windows and macOS, but I'm not totally sure. You can install docker the same way one would on a server in a qube in QubesOS and it works fine, I think I tried that once just to be sure, I just wanted to be able to have Docker Desktop. I also didn't want to paint myself into a corner in case I should need to run something else that also expects to be able to run a VM.