Teams play box and one in high school gyms all over America. This is because often times at this level there are players who are much better than the rest of their team mates and having one defender dog the best player on the other team can A) tire him out more quickly,
upset his shooting rhythm, and C) make the opposing team rely on other players.
Granted, I haven't lived in the USA in 10 years, but that's how it was for the 20 years before that.
Once I scored a basket that still makes me laugh.