A crying parent

I beleive one of the worst, if not the worst moment in a persons' life is to be the leaning shoulder of a crying parent. Parents are those people whom you've looked up to since day 1. You see them as your models, the people you should imitate. As you grow older, you further see yourself as a reflection of one of them, maybe blurred, but it is still a refleciton. You think that they are strong, they can face anything and everything.
But when things get bad, and deteriorate, and cannot hold on anymore, they collapse - as all humans beings do. When we were kids, they hide away in such moments or bury their feelings deep within. But with us and them getting older, it is more difficult. And if the issues they are worried about effect us in the same manner, then they bring it out.
I remember once my dad was crying, but his was a different weep. He was pissed to death. We had lunch at our aunts and he drank way over his limit. He went off walking through some country roads dancing from one side to the other. Me and my mum went to fetch him, and managed to bring back home. Immagine the humiliating when a passer by asked if we need any help. I wanted to bury my head just like an ostrich does. My dad then started weeping like a baby, crying out that he was going to die. Well we all do. But he was so drunk that he felt like it. That was one of the worst moments in my life.
Fifteen minutes ago, the same thing happened, same as in a weeping parent. But the scenario was different, and the person. My mum wept her worries, her concerns. I do not know what to do in these situations. It seems that all the fights and arguments between my parents were/are fruitless. People do not change, but they can change their actions. My dad is reluctant to do so. My sister is worried. I know that he does not do things as they should be done. But times change, and so do situations. Insisting on acting when the rest of the family cry to bring things to a half is a recipe of self-destruction. It shows that somebody does not care, neither feel nor give a damn.
I cannot understand why.