It depends on the school and classification. Small schools need their best athletes participating in every sport in order to be competitive. Larger schools? Not so much....
I understand that the Farragut baseball coach does not allow his players to play football. That might not be true but That's what I've heard.
Also, there are some sports that don't go together. It's rare that you find a wrestler that's a top notch baseball player.
Why? Different skill sets apply. Baseball is eye-hand coordination...
More on point, baseball players get better in the summer by playing better competition and seeing better pitching than they will see during the school year. If he is going to large AAAAA school, the football coaches expect him to be working out for football. If he doesn't play baseball, he gets behind his peers. And we haven't even mentioned the basketball coach who expects his players to playing in the high school summer league.
In AAAAA, most athletes would be relegated to being jack of all trades and master of none if they played three sports.
Thirty years ago, it could be done because football practice didn't start until August 15 and the kids actually had summers off to work or whatever. You didn't have all the Select baseball leagues. You had American Legion ball which was reserved for only the "best" baseball players in the area. You get my point....