Why cant boys wear dresses?

They can if they wish. A Scottish kilt is a good way to wear a "dress" and is seen as proper for boys/men. Society has decided that there is proper clothing for men and women. This is a gender based measure and depends on the culture. Actually, 200 hundred years ago it was men who were the fashion trend setters. High heels were first worn by men and they did wear a form of a dress. There is a wonderful movie about a little boy who decides he is going to wear a dress. See if you can find it.