I think, and have always thought, that kids are better off playing multiple sports. And even if a kid themselves only has the desire to play one particular sport I think they need to spend time away from it.
When I was growing up there was very little specialization, the only kids that did only play one sport did it because that is the only one they had a desire to play and not because they were pushed into only playing one. I played whatever was in season and just went from baseball to football to basketball and back to baseball and had soccer overlap on a couple of them. But I liked to do it and since none of them were all year round I never really got tired of doing any of them, when the season came around for each I was excited and ready to be finished with whatever one I'd been playing and get to the new one.
I never let my kids only play one sport growing up although my older son eventually quit playing football in HS and only played baseball his last couple of years but that was because he only wanted to play baseball. He actually regrets it now and wishes he'd kept playing football.
There are coaches that push kids to only play their sport, I've seen it, and it isn't uncommon. And many kids do what that coach is pushing out of fear that if they don't do it they won't ever get any playing time or may even get cut. I see it far more from baseball and basketball coaches than football coaches, in fact I've never personally seen it from a football coach although I'll assume that there are some.
And the kids fears are probably justified, there are coaches that blackball kids because they play other sports. A kid has to be such a good athlete that a coach cannot tell him no to get away with it I think. The kids that are that quality of athlete are rare, there are probably only a few of them per school each year so the majority of the kids have to conform to a coaches demands if a coach in ther school is that type of coach.
I don't know how to change it, I wish there was a way, but I don't know how to do it. It means that only the elite few get to actually play multiple sports and I think that is a shame.