The most important thing to remember about lawn darts is not that handing kids metal stakes is dangerous and stupid, but relatively few kids where actually harmed.
It's not necessarily fear, from the parents, of actual danger, it's fear of being charged with neglect. It, too, may be an unfounded fear, but until being a busybody who needless charges someone with neglect is a crime, crime and accident statistics don't necessarily enter into it.
I think you mean in suburbs. I live in New York and see kids alone all the time. I've even asked a 10 year old for directions.
I have friends raising kids in inner city areas, both gentrified and not, and they are all about getting those kids navigating the city blocks around their homes by eight or nine.
I realize those are anecdotes, but suburbs have been repeatedly indicted for a lot of the post-Baby Boomer social issues. Thus, I'm hardly surprised. Suburbs were a massive mistake: fiscally, socially, and environmentally. We're only now really uncovering the entirety of the negative impacts this sort of development had on the United States.
On the other hand, in major cities, parents are being charged with neglect if their young children walk a a few blocks up the street to go to a park. Not everywhere, but such stories crop up in the news every so often.
Another part of the problem is that communities in America aren't (or seem to not be) quite as tight knit as they once were. There are plenty of explanations, from cultural shifts, family policy, excessively punitive drug policy, economic policy, decline of religion, decline of moral values, rise of the internet and easy access to excessive amounts of televised programming, cell phones and handheld video games, you name it, and someone has probably blamed something for it.