Soldato
It would create a complete mess of confusion if they allowed some "optimized" games to run from external storage and not others. So they adopted a blanket approach that anything with the X|S logo has to be on the SSD. I think it's fair enough.
But that's the thing they haven't, SoT works fine from the external SSD and it's X|S optimised. Also a bit daft if it's just a res/fps bump as other than faster load time the internal drive gives no additional benefit. I can understand if they'd slapped on tons of new massive assets that needed to be streamed through.