Why cant they make consoles graphics so much better?

i mean i just looked at the ps3 specs and it only has 256 gddr3 vram (256 mb of graphics, nothing) so why cant they just stick a at least a GB of graphics in. because me and my brother build and awesomely great gaming PC, 4 gigs of graphics, 8 gigs of ram, 2 x 1 TB harddrives, hard core processors etc etc etc. so whats up with that, why cant xbox and ps3 make the consoles better?