How come game consoles use graphics cards longer than PCs?

The graphics on the 360 and PS3 are comparable to the latest PC games, yet they use graphics cards from 2005/2006. Why is this? And what's the point of a better graphics card if Forza Motorsport 3 looks just as good as a PC game with the latest graphics card?How come game consoles use graphics cards longer than PCs?
No, the graphics are comparable to medium/high end systems from 2006.



They are limited to resolutions of 1080p, which any recent graphics card can surpass by far. Without getting too technical with things like shader models, directx versions, fill rates, triangles-per-second and other nonsense that only matters to advertisers, let's just say that modern high-end graphics cards are way in advance of the PS3 and 360.