I have a really bad habit of just winging it when I play games.
I like to write down ten or so things that I think are interesting, and then I create the skeletons of some creatures, a list of interesting magical items (sans effects- I'll make those up later) and then some situations that the players can get it, and then I run a game.
So far, nobody seems to notice that my preparation could be mistaken for brainstorming. It's a mystery to me how people can spend twenty hours preparing material for a campaign. It's like, you don't even know what's going to happen next session. Why bother preparing campaign material for a goal you might never get to for a party that might not give two shits for a game that might not last past the week?
Of course, worldbuilding is its own pleasant exercise. But any future or former GM does that in their spare time. "What if the Etruscans sacked Rome," a person might ask, "what would such a world look like?"
Questions upon questions.