Because if you win with a lot your trainee is often subbed. If you lose though he stays in.
Like most things in this game, it punishes teams who are competitive.
Marin's stance is that it adds realism. There isn't a single thing you can compare about training a real life player and traìning one in buzzerbeater, but somehow benching players for the last minute or 2 is realism that people can't live without.