The single biggest reason, by far - kids stop playing sports because of parents who come and watch. Turns out parents think winning is important, which robs all the fun out of it.
As a counter anecdote; in high school I played 2 or 3 sports per season. I enjoyed them all immensely, despite the teams being (usually) objectively terrible. No parents ever came to watch (outside the ones offering necessary transport). We'd occasionally go a whole season without winning a single game. No expectations. Just having fun with my mates...
I'm really thankful it was my time, not my parents time.
Those activities frequently overlap. No, Cub Scouts does not care that the high school basketball game overlaps with the camping trip because Cub Scouts are not in high school. Parents are (still) not able to be in two places at a time.
This means that at least some of the time, the older kid goes to their game without their parents. (Because the younger child frequently cannot.)
If you are able to solve this for those parents, you can be the first $1B single-employee company.
People who have means sometimes are not able to see other people's contraints.
The football team has ~40 kids and basketball has ~12. The school has ~2000. At least in my area.
Small kids, of course, are a completely different story. But once they go through puberty, parents need to understand that their role in their kids life needs to change a little bit and their kids fucking need their own space with their friends.
Even if you have one child playing (or on the bench, or in the band, or cheerleading) and have come to watch, your other similar-age children will likely not be interested in sitting with you. They will want to sit with their friends, and rightfully so!
I guess it's just a generational thing, but my late Gen X mother was simply busy. Army drills twice a month will do that. I also concur with other responses that these were all social events anyway, so it's not like I was yearning for Mom to be there.