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.
University of Chicago is a pretty prestigious school, too. Tuition-wise, it’s on par with Yale, Harvard, Stanford.
UIC is half of UofC. NIU and EIU are a quarter.
OP is asking why one of the most expensive universities in the country is expensive, and assuming that all universities in the US are that expensive.