It’s because as conditions changed, they never lost that mentality.
My boomer parents make twice the food they need to eat for dinner every single time and then just throw out or freeze and forget the leftovers, they could literally cut their food budgets in half just by being more reasonable with their portion sizes.
They have coffee at home and a nice coffeemaker, they go out for coffee almost every day driving a round trip of ~50km for the exact same brand of coffee they have at home, and when I mentioned this to them the last time they were having financial problems they said something to the effect of “oh well it’s only a few dollars each” not even understanding that the gas they use and the wear they put on their vehicles is part of it too, and maybe it only costs a few dollars (plus gas etc) that when you do it every day it adds up.
It’s literally the same as talking to my preteen nieces and nephews, they just have no concept of the value of a dollar, and are completely unwilling to change a single aspect of their lives to save money, and then get confused as to how they keep running out of money before the end of the month. I know that sooner or later I am going to have to take them in, or put them in a home, because they can’t even manage their own finances and get angry and defensive any time I try to make suggestions to help them.