It's been mentioned before, but the reason you don't see kids playing on ball fields by themselves anymore is they can't. Most counties/cities lock up their fields when they're not in use anymore. They are to worried about being sued for injury or whatnot.
I know when I was playing a game of pick up baseball and I broke my ankle, I walked home, my mom rolled her eyes and that was it. No talk of how can I sue the county because my son broke his ankle. It's getting ridiculous out there, and it's effecting the kids IMO.