They are not, the problem is how you use them. For example - fetching a texture data on PS3 takes about 50 clock cycles, while on a PC it takes 1000-2000 cycles, because the request has to go through all the driver layers first. PS3 uses its own version of OpenGL(PSGL) which has a much more low level access to hardware. That level of access is simply not provided by PC hardware, so it has to be somehow emulated - and for high performance GPUs we have nowadays, it's rather difficult.
