They really should, you're right. But teachers are humans like anyone else. I experienced that in Choir class, my teacher was trying to rely on me to hold up the entire choir class, and I had to tell him, "Look, it's your job to teach these students how to sing well, not rely only on me to make up for the others."

I don't think every coach is like what the OP has mentioned. There are coaches who gives chance to each of the aspiring student but then out of it only the best players are chosen when they play against other schools. To me it seems fair enough.

A lot of football coaches like to win all there games so therefore they like to keep there best players on and there worst off.

IMO Coaches have responsibility to promote "the deserving" players/atheletes and not everyone.

BTW I have won medals in Long & High Jump as well as running. I play a bit of volleyball but that's it.

Like Raima said, coaches have responsibility of generating winners and hence its not possible for them to give equal attention to all the players.