PS3 can't handle it simple as that. Yes the Cell is great for graphics blah blah blah, but it is now a few years old and simply the PS3 is unable to handle 1080p games.
Don't believe me? Look at PC benchmarks. Only the GTX200 series + can do multiplatform games well at 1080p with AA.
With 3D coming this summer, we are probably going to see more games in SUB-HD resolutions, not more games at 1080p.