It's simple, actually. On PCs, there is no obligation to run on a lower-end PC.
Suppose a dev is trying to make its game work on 3 PC GPUs: 3060, 3070, 3080ti.
They get it to work it at 1080p 60 FPS on a 3080ti, 1080p30 on a 3070, and can't even get 720p30 (+ other game-design stuff) to work on a 3060. They'll just drop the 3060 and will put 3070 as a minimum GPU required.
They don't have that option with the Series S.
If the 3060 is the Series S in this case, 3070 the Series X, and 3080ti the PS5 Pro, they can't skip the 3060 because of Microsoft's policy. Otherwise, they will also have to drop the game on the Series X (3070) which was working fine.
P.S. This is of course a very simple example. There are other factors like memory bandwidth (and CPU) as well with the Series S.