Don't quote me on this, but I believe the reason not all Xbox and Xbox 360 games are available is because they still need some work, and Microsoft simply doesn't have the legal rights to do so without the permission of the relevant parties.
With the PS5 and Scarlett this shouldn't be a problem, I fully expect that everything which runs on the PS4 (PS4 games, VR games and PS2 games) run on PS5 day one, and the same with the Scarlett and everything which runs on XB1 (XB1 games and the select Xbox and X360 games).








