Why do stick people play on swing sets all the time?

All kinds of reasons! Swing sets are life's all-important activity!

First of all, it's fun, which is a valid reason to do anything.

Secondly, no one likes a dull breezeless day. But on a swingset, stick people can feel the wind on their happy faces even when the weather is still.

Finally, a swing set brings stick people up and down. It isn't a game that promises always up up up! It's all about ups and downs, forward movement and backward, equally. There's exhilaration in the ascent and their little stomachs turn as they drop. But stick people do it anyway, because they know that they can't have the ups without first having a down. Ups and downs are all part of one and the same movement, one and the same game.