For the record, I’m not American nor live in the US, but I have a 19-year-old son who started attending the University of Chicago this year, studying economics. Just the tuition itself is $70k. My husband and I are lucky enough to be able to afford it - I still believe it’s an outrageous amount of money to attend college.
Same reason everything in the US is expensive: we have largely unregulated, runaway capitalism which pervades every facet of life. Everything from housing to academia to health care is for profit – not only profit, but for obscene year-over-year increases in profit. Those at the top regularly make money hand over fist even selling basic necessities, and if they don’t continue taking more and more, they’re seen as failures and replaced by one who will.
The cherry on top is that, for the most parts, the citizens no longer have any real power to change any of it.
Around the same time that health care becomes affordable in the US (major hypothetical, of course), it probably means a wind change has occurred such that university costs would also be coming down. But it would be a systematic change.