'Cause we're Humans. Even if some massive disaster struck which reduced the worldwide population to like 500,000 people we'd do our damndest to survive. We cling to life just as strongly as any other species*, but we also have the brains and tools to fix things which would crush most other species. Short answer: We screw each other merrily, but Humanity as a whole is way too damn stubborn to let a little thing like several million deaths stop us.

*Sentience has created some anomolies such as depression and suicide, but even most of us headcases are about survival when it comes right down to it.