Because cars is the wrong answer. In the 60s and 70s the kids were running around the streets all day and cars then were bigger and harder to control.
(I'm assuming you mean 5-10 yo as 3/4/5 really do need supervision even in safe environments)
For what it’s worth: the right answer is still cars, just indirectly. Yes there are more factors but those factors are caused by how American suburbia is designed and evolved over the last fifty decades, which, surprise …
Walking to school is impossible in most of the US. Biking to school on six lane stroads is unthinkable especially for a child. Mobility is freedom and independence, and that independence is removed from American children. And so things evolve in exactly one direction: that of less and less freedom to roam the dangerous streets.
Parents of children in the 60s and 70s were raised with their full freedom and had no reason to prevent their kids from roaming the streets. It takes multiple generations for these changes to take effect in the mindsets of people. America will take decades to heal from its car centric mistakes… when it finally starts.
This is not true where I live in Tacoma, WA. In fact, in our neighborhood on the North End, I would guess somewhere around 40% of kids walk to school. There is an elementary school two blocks away from me, and a middle school five or so blocks away. The private schools are a bit further and would require driving or cycling.
The neighborhoods I used to live in the residential areas of the Seattle core(Madrona,Leschi,CD) were similar.
Either way, are you seriously going to sit here and argue with someone that a 4 hour walking distance to a school is somehow normal? Even in the US? I’m sorry dude but I’m done talking to you. I hope you enjoy the unique place that you live in.
Even if we ignore the threat of being run over, constant noise, pollution, less space for walking etc
Cars create a vacuum where if you were mugged in plain day light next to a motorway everyone knows the chances of being helped by a motorist is incredibly low
But if there are more pedestrians around you, the chance of getting help from a stranger goes up
Cars create dangerous deserts everywhere and that's why we don't let kids out
One is more likely to be mugged in New York than in a Kansas suburb. Long before cars existed, people have been terrorized by highwaymen on country roads with little help or recourse. If anything, cars have likely reduced muggings as fewer people on the streets have created fewer opportunities to be overtaken and robbed.
Or maybe a large clump of people makes everyone more likely to experience the bystander effect.
Car density has increased a lot in many of these places, as has average speed (due to widening efforts, it seems). They are also bigger and also heavier on average. Or at least weight has been increasing since the 80s. It did dip a bit earlier from then, as the large steel frame cars shrunk due to gas prices. Visibility is a mixed bag but is often worse now in the front (cabs are much higher), which is pretty critical for the kids-running-around scenario. Pickups are especially egregious in this way. They are much bigger, much heavier, and have terrible visibility these days. They are a growing segment for reasons unrelated to their nominal function.
A friend recently commented after visiting his childhood home (parents still lived there). He spent the Sunday evening on the front porch, an recalled when he was a kid they would probably have been playing street hockey all evening in the neighbourhood, but that would be impossible now.
Plus, cars today are actually heavier IIRC, and faster.