This, and also other factors like a curtailed salary contributing to the increasing debt:salary ratio.
I have friends who are doing well and some who are doing badly. They all work pretty hard as far as I know. For some of them, money won't take care of itself if they don't manage money well.
Graduating with >400k debt should be daunting - if it isn't, then either you have your parents paying it off or you already have an approach on how to pay it off. Opening up a practice has become so uphill, dental service organizations (DSOs) are even becoming way more lucrative. With an interest rate of 5%, that's going to be an additional 20k the first year you have to chip off before you pay off the principal amount. The real climb is the first 5 or 6 years after dental school when you have a starting salary and high interest.
Those who live frugally after school and pay off their loans aggressively will save many years down the road. It's hard since the first thing you want to do after being in school for 12 years is to treat yourself, but you have to manage your finances very well if you want to pay off your loans fast.