It's pretty hard to admit that you are wrong. Especially if you have an ego problem. It's because you are admitting that you are flawed, and your flaws have negative consequences. That's hard to own up to.
There is also another saying, that if someone apologizes more than once, they are trying to protect their damages egos. Apologizing makes people look like a good person, in a weird twisted way.
I've been known to do both of these, so I understand where people come from when they struggle to apologize the right amount of times. I also understand how frustrating it can be not to get the satisfaction of getting a good apology.