Because its an easy thing to do, a cop out. Lifes greatest temptation, just much more chemically available now.
Oh yeah, and because focusing on all life's shit distorts the happiness and joy we had as children, and the good that exists everyday. Thats a really tough excuse to rationalize away sometimes. But occasionally we get to play, experience true extacy and nirvana, something many threw away whilst growing up, and while there's no replacement for that joy we once had, except perhaps love, drugs can make the reality of how we let our sense of wonder and excitement shrivel up a bit less painful in the moment at hand.
At my core, my drug abuse stems from disappointment at letting that happen. But i see that in so many people, drug users or not. Its terrible and beautiful at the same time, that losing something so special can do this to us as a society collectively.