I didnt have prom at my highschool, just 4 big parties every year. The entire second week of school is nice too, they turn the cantina/main hall area into a dancehall and Dj's come playing stuff during breaks.
1) That is a reason
2) I just dont' find school dances (prom, homecoming, etc.) to be any particular use to my time. It's also a waste of money to me.
3) I'd prefer my own personal prom, one where I choose the music, I invite the people, and it's free to a degree.