And because some folks have kids first, then find out how horrible it is. The first child doesn't always seem horrible, so they have another.
And because even happy parents find a lot of the work involved absolutely horrible. I'm not sure if anyone gets joy out of waking in the middle of the night to feed the baby, change an overflowing diaper of shit, and/or deal with spit up and so on. Older children keep doing pretty disgusting things for years to come.
Even jobs taking care of folks are considered low-rung, crappy jobs with horrible pay.
Parenting thoughts from a non-parent - pass.
Edit: Besides, me being childless is exactly why I'm qualified to answer part of this question. I understood that, for most of my life, I had to hope that the birth control methods available to me didn't fail (basically condoms, since I can't tolerate hormonal birth control and couldn't afford to be sterilized, if I could even find a doctor).
Its equivalent to taking relationship advice from the neckbeard down in his mother's basement.
And judging by the fact that women have rushed into the work force while men have not rushed home, I do think it's fair to say that people would rather be sitting in an office than chasing a toddler around the house.