Adult playgrounds?

Why are there only children's playgrounds? I mean, why are there not adult playgrounds. I am not talking about adult playgrounds in the way the word is used in adult bookstores or adult toys, but specifically places that you can go and swing, slide, climb, jump, fall and generally just have fun, preferably for free. It seems that unless you have kids yourself, and they are actually present, there is something socially unacceptable about adults playing on play equipment (even if there are no other children present).

Not sure why, but as I used to say "that's not fair".