I can't speak for others but at least for me, I have an irrational desire for a family. I'm well aware of the costs, the setbacks to your career and other penalties but at the end of the day my attitude is: "I don't give a shit, I want a family"
I can't speak for anyone besides myself, but my anecdote is: I was a DINK (Dual-Income; No Kids) for a long time. Both the wife and I earned well. I thought I didn't want kids (I didn't not want kids either). Wife got preggers, she quit her job to be a SAHM. This cut our household income in half. We have to live on a strict budget now, but, given the option, I would never want more money and fewer kids. I want many more kids.