It’s a Goldilocks thing.

The number’s just right.
I mean, four and five are good (and popular), but seven gives a writer more to play w. and fans more to enjoy, without being TOO much.

There are exceptions of course, like the Satellite era League, and Oceans Eleven in movies.

Speaking of movies, Akira Kurosawa‘s SEVEN SAMURAI (and its American adaptation) really put that number into the (I think I’m using this right) zeitgeist.