It's because that crosses the line of plausibly legal. In theory you could use an emulator with only titles you copied from your own physical copies which is legal. But if they implement a download mechanism it's clearly illegal.
At any rate you can replicate the same thing by just hosting the ROMs on your own cloud storage and using something like macos virtual files which will do this transprent download/delete to manage storage.
At any rate you can replicate the same thing by just hosting the ROMs on your own cloud storage and using something like macos virtual files which will do this transprent download/delete to manage storage.
You can make an archival copy, but it is copyright infringement to use that copy in an emulator.
For newer console generations (decrypted) backups are not legal due to needing DRM circumvention.