They don't want to overuse their first party titles, even if it is the same game (minus the lower graphics).

They need to properly organized their first party titles so they can milk them with sequels as much as possible, they can't just port every game- people will get tired of them (one way or another).

Nintendo is the master of this but they don't port their games much neither, they just make new games with the same characters.

Gran Turismo on Psp = Boner, played the hell out of GT1 and GT2, port them and pump up the visuals to premium PSP levels! it would sell millions in japan alone ( where psp piracy is less... i think lol ) come on sony milk the Gran Turismo cash cow!

I doubt the PSP can handle PS3 ports, maybe some PS2 games sure, if that's what you mean.