Over the same time period that loans have become easily obtainable (via US government backing), massive amounts of regulation has also been imposed on colleges/universities.
While people love to talk about football stadiums, luxury dorms, and other amenities, one of the largest increases in the cost of running a college is administrators to keep up with regulatory demands.
I'm not claiming administrators are solely responsible, only that it's not as simple as "Uncle Sam gives students money and college spend every penny".
Additionally, public funding of colleges has dropped over the same time period. As of 2018, state funding of public colleges was down ~$7 billion since 2008. It was slashed during the down economy, but never brought back up again.