It’s a common idiom. And idioms are often weird. But that is exactly the point.

> Approximately nobody “flies for fun” under that tortured definition.

Yes. That is exactly how it’s supposed to be used. The vast majority of people do not enjoy the airport/airline experience and were there any other reasonable way to get where they are going they would probably take it. They are not doing it because it is fun, they are doing it because they have to.