Because if your hardware doesn't suck much of that that ridiculous power the PSU will be pretty inefficient. Most PSU's show efficiency only when they are over at least 50% load.
"There is enough on earth for everybody's need, but not for everyone's greed." - Mohandas Gandhi
Dude, I have a GTX 260. It needs 22.5amps and 550 watts. It's the most demanding part in my PC.
I run everything in my PC fine with a 600w 27amp power supply.
Thermaltake :3