Because having children is somewhat natural. It's a logical step in adulthood. You reach a point where you think you've learned tons and you want to pass that wisdom on to someone else. Of course in reality you don't know jack $hit and that's what the kid's gonna teach you but you don't know that going in. Children bring you out of yourself. Most parents will sacrifice a great deal for their children. Suddenly your own well being comes second to the well being of your child. I don't see "selfishness" as being a big part of parenting.

If you don't want children then clearly you shouldn't have them. But if you're gonna spout off like a holier than thou a$$ claiming you're not having children in order to save the world while the rest of us are selfish breeders hell bent on destroying the planet you're gonna get laughed at.