Same reason why it happened in other countries before us: scarcity of resources and over-population. Want to see world population and scarcity effects in a microcosm? Go to a nightclub. People get really fussy when the line gets too long to get in. But when there's no line, everyone's like, "Oh hi! Hey, how are you? Man, it sure is empty, let me call some friends!" We don't vary much in behavior when it comes to non-nightclubs that get crowded.