Because +12V rail is not only used by your graphics card, but other components as well, and this might exceed 36A without any trouble. Even if that doesn't happen, the PSU will run maxed out most of the time while gaming, being under heavy stress (compare it to driving your car with full throttle all the time - this cannot be good for the car, right?). Even the best quality PSUs should not be run like that. And this one is cheap, meaning it will likely fail. Soon. And when PSUs die, they can take other components with them.
You definitely need headroom here, and a quality PSU.