Those things are given up because parenting-time is up 20-fold from a few generations ago.
From the 1960s back, kids needed parents a few hours a week.
But we reduced kids' roaming area from many sq mi to just their own property. At the same time, we instituted 24/7 adulting. Most of those hours are filled by parents.
Kids have permanently lost daily hours of peer-driven growth - the ones where complex social interactions occurred naturally. Parents are now left with trying to construct artificial environments (leagues, programs) where maybe some of that can occur.
Those efforts eat time and resources. And they're a poor substitute for the vital environments that kids once had for free.
I spent 20x the time parenting that my mom did. For all of that, my kids had little-to-none of my growth opportunities.
And of their infrastructure where everyone wants to have their own suburban kingdom with large back yard, swimming pool, garage for two SUVs and workshop.
Americans don't like living in small apartments like Europeans and Asians so this is what they get.
"This is what they get" is false causation, these things have been present for decades. Helicopter parenting, liability for walking around alone, special snowflake treatment are all newly-introduced ideas.
A few generations ago, the trend was that US kids had a broad range of appealing places to roam and were generally free to do so.
Presently the US trend is that kids have few if any appealing places to go. And should they roam anyway, they and their parents risk legal (and increasingly permanent) consequences.
I suspect there are no other kids outside because there are few/no desirable places for kids to congregate - places that are safe from moving cars, enforced property laws and adults with poor judgment.
The roads are also twisty & turny. They're difficult to drive fast on.