You know, I was thinking about this recently.
When I was young, we lived in Oakland, California. There were at least four playgrounds within walking distance. I recall going to them with the sibs; I recall nothing about our own yard other than that it had a lemon tree (and that because of a popular song at the time which alleged they were “impossible to eat”. And I knew that was not true as I did it all the time. This was in my mind yet more evidence that adults regularly persisted in saying that things were true which obviously were not).
When I got older, we moved to the suburbs of Atlanta, Georgia and we had a jungle gym. I kept wondering where the other kids were because in my mind a playgroud=where other kids are. My father explained that this jungle gym was ours, in our yard, and other kids would only come if we invited them. This was also sort of cool, like having a circus of your own or something.
Now I am aged and have my own kids. When we lived in Atlanta, Georgia we had a jungle gym, a wooden set we got from a neighbor who was removing it as her kids were grown up. It was quite a nice one. My kids who were then 4 and 2 played on it a lot.
Then we moved to Holland and there is no room for a jungle gym. We live within walking distance of four playgrounds so we just go to those. The kids are now 7 and 5.
We go to the States every summer and last summer we stayed with my sister who has a jungle gym, a really elaborate one with a climbing wall and two slides and god knows what all. And Eldest played on it often but then asked me where all the other kids were.
I thought of a jungle gym as a public space; my children now think of it the same way. It seems to be a function of living in a city where it is likely to be a public space. My niece, she of the elaborate-jungle-gym, clearly thinks of it as a private space, it is her own domain. They like the aspect of the chance of running into other kids; she likes the aspect of its being hers, a place she can invite her friends and have a good time or just play by herself. (Well, not really by herself, my sister won’t let her use it without supervision. So she can really only use it when we are there or when my sister has the time to come outside and watch her. Now there’s a waste in my opinion. But that’s a whole 'nother thread.)
I think the adult cognate is exercise equipment: a lot of people get very elaborate stuff and install it in their homes. Some of them use it a lot but in many cases it ends up collecting dust. It seemed like a good idea at the time. Put the same equipment in a public space – a gym or similar – and a whole different set of people use it a lot, because for them exercise is a public event, not a private one. And some people sign up for a membership and never use it at all.