Those are brought by a recompilation process almost like porting rather than full system emulator. This is why not the entire library is available.

Sony could do this too but they do not.

Technically, they (Sony) have done that with the original Playstation. POPS (PlayStation One Portable Station) is their software emulator. When the PSP was hacked people were able to inject PSX titles to it, but it wasn’t 100% perfect. Some titles couldn’t load due to multi-disc (which Sony implemented later for FF7, 8, 9 and MGS), or just couldn’t load at all due to not being programmed for the particular emulator calls.

PS2 and 3 could probably do that, but it’s a MONUMENTAL task to do.