Cause it WORKS. Thats why. If your goal is to be 250 lbs, and your 200lbs, you need to eat like a 250lb guy. Dante said that the athletes with the most muscle mass lb for lb are sumo wrestlers. Do they train with weights? Most do not Im sure, but what they do is eat tremendous amounts of calories. And yes, they are big bulky bastards with high bf% BUT under that fat is an extreme amount of muscle.
Im not going to flame you, but there is a reason that you stay at a very consistent bodyweight year after year. You are the first to write those threads about genetics and drugs and so and so. The missing link for you is the fact that you probably have never done a serious bulk. You stay the same year after year, 5lbs here, 2 lbs there. If you want to grow, you have to EAT.
There is a reason you see guys, alot pros even, stay basicaly the same every fucking year with only a couple of lbs gained, its the diet in my mind.
How can anyone expect to be a huge massive beast if they keep eating the same way year after year. There is a time and place for a clean diet, but if your goal is to put on serious mass, its alot about the fork to the mouth and repeat repeat repeat...