They're called cars. Houses are packed tighter and there's more cars per household than when I was growing up (maybe due to everyone being double income now). Streets are also narrower and most have street parking, creating visibility issues. Go check out a development than went up 40-50 years ago compared to one that went up in the last 5 years. The difference is pretty stark and pretty hostile to kids running around doing kid stuff.
I don't think media's focus on bad guys has nearly the impact that the enormous increase in cars has had.
We live a block from the playground, close enough that I can almost see it from our window, but you can't get there without crossing the street. So our kids (7y, 5y) can only go there with a grown up. I've worked on teaching them how to cross the street safely, but they're just not good enough at checking for cars yet.