I grew up in them Scandinavian countries, and my brother and I often spent the entire weekend outside out of sight from my parents. My earliest memory of that is when I was 7 years old.

I now live in the US and have kids about the same age.

The issue is not homogeneous culture or any other thing people with no brains come up with, but traffic density, at least in my case.

Where I used to live we could roam without having to cross dangerous streets. Where I live now, there's traffic everywhere and cars don't give a shit about people on foot, so my kids do not roam.

When I was a kid, we used to have vast areas to play safely. I just looked on Google Maps and the cross-country ski route we used to regularly take on weekends is 4 miles long. We had to cross ONE street. I used to bike about 10 miles one way, by myself, when I was a little older and keep on bike lanes the entire trip.

We also used to have large play areas in the backyard of every home I ever had. The bigger ones were the size of a typical New York borough block. We didn't have to leave our homes to get to a playground, or a park...we had one right outside of our door.

Obviously this sort of thing is easier to do / plan for when the entire country has less people than the city of New York.

Forgive me, but it seems that you're taking a common argument (X policy that works in Scandinavia won't work in the US because the US isn't homogeneous) and extending it to some sort of xenophobic dog-whistle (i.e. kids in the US can't roam because the US isn't homogeneous). Is there a more charitable way to interpret your comment?

The US hasn't become substantially less homogenous in the last few decades, so that's not it.