It's because the the $100-400 a month that the normal person is paying per month often doesn't crack the principal. you are just paying off the interest accumulated. And for a lot of millennials the interest accumulated was quite high during the recession and the years that followed. If you are earning 35k a year (which is what the average millenial makes) and your rent is 750-1.5k (depending on city and roommates), it can be pretty difficult to make a dent. Obama's income-based repayment plan has actually kind of added to the problem. 10% of 35k definitely doesn't make that dent when your interest rate is 5-7%.