Because they saw what the world could be with new people in it because they were once new themselves. They knew this world was chaotic and if other humans and creatures were going to get you, Mother Nature and Chance would. Still, they knew you had a chance and even cleaned up the world a bit to make it easier for you.
It's not perfect but it'll be good enough when you get here and better when it's your time to pass it on to your young.
Because if our ancestors can handle near extinctions, saber tooth tigers, and 10 ton woolly mammoths then a bunch of angry people would be no problem.
There will always be chaos but not having babies would only deny the problem solvers.