The answer as far as I can tell for most people is that they didn't actually choose after careful consideration of merits. They either just kinda felt like they wanted kids or were told that they needed to have kids or kids just kinda happened, or some combination of the three. But so much of the terrible part of having kids is hidden from potential parents that few people ever get to make a well-informed decision.
Have you ever looked at someone with young children? I mean really looked them in the eyes? They look miserable. Every single one of them. They deny it up down and sideways, but it's true except for rare brief moments. It's just that the rare joyful moments are they ones they remember because our brains suppress trauma. And it lasts for years; that miserable look doesn't go away until they've burned all of their youth. Do they love their children? Yes. Would they do anything for their children? Usually. And so would many pet owners. Are they proud of what their children grow into? Mmm...sometimes. Are they glad that they had children? Mmm...sometimes. Would they do it again if given the chance to turn back the clock? Mmm...sometimes. That's the dirty secret that you're not supposed to talk about.
Not having children makes everything less stressful, and by everything I mean literally every last little thing. Every single moment of every day of your entire life is cheaper and easier without children. Every decision you make involves fewer obligations. Want to DuoLingo for a couple months and then burn some cash and move to a foreign country for a while? Go for it. But good luck integrating an 8-year-old into elementary school on a one year tourist visa.
My mother and father had a bunch of amazing life adventure stories. None of them were from after I was born.
No regrets so far.