While the majority of people in this thread have already stated the obvious reasons why console graphics will never surpass PC ones, I would like to provide one that doesn't occur to most people...the graphics are already too good.
Some people may laugh at this statement. But consider what has been happening this generation. The PS3/360 have been suffering due to a low adoption rate. PS2 outsold or sold just as well as it's successor for many years. There are no plans for successors anytime soon. And the Wii has experienced booming success.
All of this points to one conclusion, the elephant in the room that the industry has been trying to ignore.....that graphics have reached the point where they can no longer serve as a selling point. The PS3/360 are already overshooting the market as it is....better graphics will not solve that conundrum.
To be honest, though, I actually think that the slowdown in graphical advances is beneficial to PC gaming. While PC's have always had the most advanced graphics, the constant push for improved visuals has had a tendency to make computers obsolete very quickly and thus leaving the majority of the market behind (most consumers have no reason to upgrade frequently; afterall, they usually use their computers for simple tasks and not gaming). With graphical advances slowing down, it means that PC's capable of modern gaming will become more affordable, slowly increasing their number and thus expanding the potential market for PC games.