Because we want to feel good about ourselves, in such a manner that we feel superior. In order to do this, we must improve upon are ablilties, both physical and mental. We all strive to be better, to be stronger, then everything else. I mean, look at our scientific name. Homo sapiens. "Thinking man." We say that as if nothing else thinks. we want to be better, to have something to be proud of. We are fools, destroying the planet, poisoning the very air we breathe, sometimes killing millions for dictatorship. I have seen tallies of those lost in WWII, but not the countless trees and animals murdered to supply the war effort.
In a nutshell, we do this to absolve ourselves of blame, and to feel good.
EDIT: Sorry if this makes anybody feel bad, that was not and hopefully never will be my intention. And, sorry if this makes no sense.