Because they where older. They had fought in wars. Worked in factories at 16. Got married and had kids at 18. The may be the same model year but they had many more miles. The question may be better asked why do young people look and act like overgrown children.
They seemed to dress ‘older’. Suits, ties, slacks, short groomed hair.
Now even seniors wear loose shorts, sloppy tee shirts and sandals. And walk around like bums with half grown beards.
By the way, I’m guilty of some of those. Society sets what is acceptable dress. In those days ‘real’ men didn’t wear shorts. Women didn’t go out with doing their hair (or wearing a hat) and tattoos were for low life Carneys or sailors.