Do you know, it seems extraordinary, but sometimes people just don't know any better.
Their behaviour will not change because they have nothing to model it on. Every influence that they have been subjected to has taught them to behave the way that they do.
The examples that you give are very wide ranging, the reactions of the people involved will be influenced by many things.
But the 'feral yob' comment, sometimes people are subjected to such harsh treatment that they withdraw from normal social interaction.
Their lives are so full of fear that there is no room for empathy, or compassion, just a struggle to stay out of danger.
It makes people 'hard', and they are afraid that if they let their defences down, it gives people a chance to hurt them.
After a time, it becomes normal to 'get in there first', to protect the little they have, and the little they have, might be their very survival.
I think it comes down to fear. The pregnant woman is afraid of having nowhere to live, so they brazen it out, and say 'well, it's not my fault I can't pay the rent, let the landlord deal with it'.
In an ideal world, I'm sure they'd seem to be 'nicer' people if they had plenty of money, and no reason to fear homelessness.
People are funny, and some are in a better position to deal with everyday life others.
I don't know whether there is a difference between making allowances for people and making excuses. It might just be a question of perspective.
If you are fortunate, you can just walk away from people who are making your life unpleasant. If not, trying to understand their actions might make it easier, or make you feel better about stuff.