I have a kid, at some point we give up trying to keep everything tight because the little devil would immediately make it a mess again. Now we do the bare minimum.

That's the explanation.

Agreed, at some point it's just too much following a three year old around and cleaning up. We just tidy up his play room once a week and try to instill a bit of "keep it tidy" in him with some success.

I've heard it described as "like trying to brush your teeth while you're eating an Oreo".

This may be observer bias. People who have no children and a really messy house may just visit their friends instead of calling them to visit.

My theory is that people with no children whose houses get messy have the disposable income to throw the old house away and just buy a clean one.

People with children have enough sex to reproduce but not much sex generally speaking. Ask anyone with children