I suspect some of them are genuinely happy they had kids, some others are lying to themselves, and some others lie to others because they don't want to become pariahs when it's found out that they wish they hadn't had kids.
You can read the opinions of the last group on some anonymous discussion forums; they're real people.
What the relative ratios of these 3 groups are, I have no idea, and I doubt there's any way of finding out.
A child is a life long decision. You don't get to say "oh I was wrong" after a couple years.
Parents leaving their partner and their child behind isn't unheard of. Even women sometimes do so. Sometimes, children end up at the grandparents, and the woman tries to start over. Your claim seems to come from an idealized world.