Which is only true if RSX were in a PC. However it's not. For example, on a PC the devs don't know what hardware will be in the PC, so just produce their vertex and pixel shaders and leave the hardware to work through it as fast as possible. If you get idle cycles on the vertex shaders, they're just idle. US get around this problem. However on PS3, because the hardware is known and devs can measure exactly what's going on, they have the option of using idle vertex shaders to do GPGPU work, or anything else. They can seek to shift workload around the available execution units. Then there's the matter of getting RSX to work with Cell. We've seen and heard of marked improvements by using Cell to pre-processor graphics. These techniques weren't available Day 1, and are still in their infancy regards the life cycle. What devs could get out of RSX in those first years of development is well short of what they can manage later on.
I think you just plain underestimate how complex these hardwares are, the limits placed on devs, and the scope for human ingenuity. Even known quantities can go on to do extraordinary things when devs get a chance to explore and play and really push the hardware.