Because old people do, and the most unconfident young people in the world desperately want to act like old people so people will take them seriously. They never stop to think why old people act so stupidly.
Old people look down on young people because they don't understand that old people are the reason why this generation has it much harder than their parents ever did.
"You're twenty, and you don't have a big house and wife yet?"
"No dad, I'd have to work for twenty more years years at my minimum-wage job before I'd be able to afford a house."
"Nonsense, those are just excuses! You're just lazy!"
https://www.youtube.com/watch?v=BZJDTod_fXs