Because there are higher cylinder pressures in a forced induction motor than there are in a naturally asperated motor making the same power.

Also because it's ridiculously easy to swap a pulley on a 500hp motor and make it a 600hp motor -- it would be in GM's best interest (in terms of their reputation with performance enthusiasts) to make this theoretical supercharged 500hp Camaro a lot sturdier overall than the theoretical NA 500hp Camaro.

At least, that's how I look at it.