At least in our little town, it used to be "do everything" was the norm.

You had soccer for a few months in the fall then it ended and you had basketball for a few months then it ended and then you baseball / tee-ball for a few months. And most little kids did all of the above.

At the highschool level you still had many multi sport athletes who played several sports. Coaches knew that and expected that they might not get some basketball players until the football/ soccer season ended

Now... soccer is year round. Baseball is year round. Basketball is year round. And so you can't do all of them. So numbers are down by a third because the club system demands that you commit all your time or nearly all your time to one sport not three

And it's true of other activities as well. Robotics club is nearly year round. Drama club is year round. Etc etc etc

The club system demands full time commitment much much earlier in a kids life now, which makes them not able to do the other sports. Dropping numbers in every sport because they don't have the second or third sport participants anymore