Sounds crazy as hell, yeah, but bear with me for the theory section here...

What we need is:
a) high protein, low carb, high calorie
b) properly loaded out w/ vitamins and minerals

...and we invariably manage to screw up at either a) or b), either not getting enough calories, too much carbs, or vitamin/mineral-deficient. We try to mess around w/ diet regularly, looking to formulate something that's got enough energy and vitamins to sustain us, but something usually just doesn't match up.

Meanwhile, cats eat the same crap every single day for their entire lives and thrive pretty well. And it just so happens that cats cannot metabolize carbs properly, so their diets must be high-protein. Also, since cat owners cannot be bothered to supplement their pets' diets with sources of vitamins and minerals, decent premium cat foods are made to provide everything cats(*) need.

*AFAIK, same mix as us, both being mammals and all. The only exception I know of is that cats synthesize their own Vitamin C, so that would need to be added from an external source if an all-around cat food is used for human consumption.