I think your citations are instances of confirmation bias; also, you're only citing a handful of examples, and they're examples so extreme they made national news. (They're also each a year apart.)

Even in large cities kids roam alone in their neighborhoods and parks. I live in SF, and regularly see kids under 10 skateboarding or riding their bikes on their own around my neighborhood. Now that I'm a parent, I also see toddlers running around fields in GG Park, barely within in earshot of their parents.

Odds are, if you don't see kids running around your neighborhood, it's either because you live in a neighborhood without many children (like most of SF), or you tend to be at work during the hours in which most children play.