Because they're vector processors! They're designed for number crunching, they're not general purpose cores like Xenon's cores. You can't just say "hurr durr, PS3 has more processors than the 360 so its better."
Now, in GT5 vs Forza 4's case, it's very possible GT5 has the advantage in physics calculations due to the nature of the SPE's, but that doesn't make this comment: "Oh dear...So i guess those 4 extra processors means shit then. More is less." any less ignorant.
Also, you're not helping with your "I know all this stuff and you don't" attitude.