Most of the debt free people I've met in my life, were single.
When I was single, I too lived a debt free lifestyle. Upon marriage with no kids, it was even easier with both people working and sharing household costs.
However, once kids came along, all the dynamics changed.
Now there are some people that have kids and don't incur debt and that's mainly because their relatives are helping take care of the kids while they're working, but if you don't have that kind of arrangement or family dynamic, then you're paying childcare and/or pre-school and/or a nanny.
After our 1st kid, we did the math and financially (due to tax reasons) it didn't make sense for my spouse to work as that salary put us into the next tax bracket and we'd have to pay someone to take care of our first born child.
Yes, there's a deduction for childcare, but it doesn't offset the costs - not even close.
After the 2nd child, the costs for 2 children in childcare or pre-school skyrocket. Suddenly you're looking at around $20k.
Our neighbors, both parents making right around six figure income each, got a nanny and the cost of that nanny was over $30k/yr. plus they had to provide her a car.
Throw in the mortgage, two cars, and all household expenses and there goes the debt free lifestyle.
Edit: Forgot to mention that there is one way to get out of debt forever...... and that is if you die.