For the same reason I don't bang my head against a wall for fun.
When my Ferret and found our own place, I was relieved. Her folks were very nice to let us stay there while we got it together, but there were 7 people, 1 bathroom, 3 dogs, and 2 small children.
I don't like kids. I can manage them, in small groups. I tolerate them. I occasionally even think they're cute. But seeing a baby in a stroller does not make me wish for one myself. I seem to lack the maternal imperative. Obviously, we need some around to continue the species; obviously, some people find fulfillment in having kids. Don't get me wrong- I'm happy for those who're happy with kids, who are good parents for good reasons (I can't define "good reason", but I know all the bad reasons).
So, no kids for me. The world does not need my parenting skills- any kid I unleashed on the world would be a world-conquering tyrannical despot or a constantly-stoned couch-philosopher. It's safer this way, really. And I don't need kids. I have plenty of walls handy, thanks.
What drives me nuts is all the people who smirk and say "Well, you're young. You don't know what you want. Just wait til your biological clock starts ticking. Who'll look after you in your old age? Don't you want a family?" Any or all of the above. Like I haven't thought about this for years. Like I'm not capable of NOT wanting kids. Like I don't have reasons for not wanting them. Like I haven't thought of all this before.
I have a plan for all this. My plan is: Have my finances set up to take care of myself in my old age. And have friends, who'll visit me in my dotage. If my biological clock (which I'm not convinced exists) starts going "cuckoo!" and I start wanting kids around, I'll join a mentoring program. Oh, and as for "family"? I have a family. Besides my immediate family and the family I'm marrying into, there's also my own family: me, Ferret, and our fuzzies. We're our own family. Small but happy. Yay for us.
I wish people would consider adoption more often, though. I know there're all kinds of problems with the system- one of the biggest that in many (most?) states, LGBT folks can't adopt. But I think the biggest problem is that we're still stuck in this medieval mindset where kids were a necessity (for work on the farm) or a point of pride (look, my penis is manly and virile!) or a point of fear (the family name will die if I don't have kids! [never mind that usually a woman's family name disappears into obscurity]). These, and all the other stupid excuses to BREED, DAMN YOU! don't make sense to me.
Hey guys? Screw your genes. The world doesn't need them. If you're worried about your race or religion "dying out" or "being outnumbered", you're a bigoted asshole, and the world REALLY doesn't need your genes. And any talk about "bloodlines" is ridiculous. This isn't the Middle Ages, you aren't European Nobility. In this age, paternity's no longer a life-and-death issue. Let's see, what else? Most of you don't have farms; if you need help, hire it. Your penis is not that damn important- no, really it's not. Nobody cares. And unless you or your ancestors have done something particularly noteworthy, even your descendants won't remember you, and you won't know them. If you did do something important, you'll be in the history books anyway and you'll be remembered, no kids needed. If you're that worried, make a couple million dollars, endow or donate something big, and give it your freaking name. There ya go.
I can't tell people what to do. I can't suggest we all stop reproducing (although I think we could benefit from a slowdown for a couple generations). I can roll my eyes at people with families of 5 or 6. Don't you guys know when to stop? Think of the homeless kids you could provide for! Hell, think of your carbon footprints and stop making more consumers! Damn, people!
Think. Adopt. Use protection. And for Gaia's sake when someone tells you they don't want kids, congratulate them!