Probably the biggest reason people don't say "I'm sorry".....is because they are NOT. You can wait around for your deserved apology, or imagine that "they ARE sorry, but it is just too difficult for them to voice that sentiment." Deep....deep down inside, it is probably their pride (you conjecture).... that keeps them from uttering those emotional and empathetic words .....those words that will evoke true forgiveness....cage animosity ...and prove love and humility. But you never hear them say it.
But the truth is that the biggest reason people don't say "I'm sorry"......is because they're not!