Play structures are so easy that even children can figure out how to use them correctly. There is a wheel. Spin it! There is a slide. Go down it! There is a staircase. Climb it! There is a tunnel. Crawl through it!
This can be good – sliding down slides is quite fun, and crawling through tunnels could entertain a six year old, and I guess spinning a wheel is nice.
The issue with play structures is that they train you to play only within affordances. When the space that you play in is designed by someone else, you are pushed towards playing only in the ways that they envisioned. A play structure affords a small set of actions, and as a player, you only get to pick which one you prefer. There’s no invention in that, no spontaneity, no creativity.
Let’s think about how this applies to adults. If you show an adult a library of video games, they can pick one and click the “play” button. But show an adult a meadow with trees to climb and grass to roll around in, and they will write it off as unplayable (unless they are with friends, and typically playing a standard game with standard rules).
Writing code, painting and most other hobbies are forms of play that don’t come with an affordance. Doing those things requires that you drive yourself. You decide to do them out of an infinite set of possibilities, filtered by your own desires! And that is a good feeling, much better than letting someone else decide (either through the design of a play structure or the design of your society) what you are going to do today.
People who don’t have hobbies usually don’t know how to play without affordances.