The first few lines of this article sums it up completely as to why.
"Between misdirected payments by one of the companies servicing his loan and the abusive collection tactics he encountered when he fell behind ..."
These companies that bought the loans on cheap from the secondary market, make more money from penalties they they do from loan servicing. And they have as many tricks up their sleeves as they can discover to help make that fact true.
They are the true sharks in the whole equation, with guaranteed support from the government and the tax payers.
I recommend that every last student default as things are now. If I were in charge of writing new laws for new student loans, which I am definitely not, I would allow them with three additional conditions. 1. They qualify for personal bankruptcy like any other consumer or investment loan, 2. they could NOT be sold off but would be held in a public trust fund, initially funded by public money, and topped up as needed, and 3. (Scandinavia does this) make repayment a modest surcharge in one's income tax. Make a lot, pay a lot and pay off fast. Make a little, pay a little and maybe never pay it off.