It's because they don't want to sacrifice a fun lifestyle and material items to be a responsible adult. This comes from my experience employing 20-something's over the last 10 years. I call my crew the "lost boys" like in Peter Pan. None of them want to save a dime, live paycheck to paycheck and stay with their parents but will buy the latest iphone, gaming console, weed, etc.
It’s funny, the only people I see in my office buying new cars, taking private jets, and constantly complaining how broke they are, are all over 50.
Us 20 something’s are working 70 hour weeks and don’t have to spend our bonus to make ends meet.