If it is true that a college degree can pay for itself in a few years after graduation, then the amount of demand for college degrees would rise (it's a no brainer to do it right?).
But the production of college degrees is somewhat limited by the availability of positions, and is not infinite. Therefore, if demand rises but not the supply, the cost _must_ rise to match. Therefore, after some time, the cost of a college degree slowly grows to match the potential future value of having that degree (fuelled by free debt).
I think at some point in the last couple years, the cost of a college degree has, on average, grown to almost match the future value in earnings accounting for the debt repayment.