Have you ever seen Superbike 2001 at 1024x768x32. Trust me, if you were to look at a PS2 game on a monitor, it would look horribly blocky. TVs resolution is 640x400 i think. And probably the games are running at 16bit color.
Another reason is that the companies that make Console games (like Sony, Squaresoft, Namco, etc.)have a huge advantage in terms of money and staffing. It would probably take a normal PC development team twice as long to make a state of the art game for the PC as it would for a console. PC technology and software are always changing. New DX versions come out, new graphics cards with new capabilities. Consoles represent a static environment. Sony tells developers what the console can do, how to do it, and the developers only have to implement these findings.
Just as with PS1, in about a year, PC graphics will be looking much better than the PS2.