Because they can. If they wanted they could make it more affordable but there is so much wastage of resources and unnecessary expenditure. In my time at University of Minnesota, perfectly good signs outside each office and exit signs were changed at least 2 times in 6 years. I have no idea why they were changed. Basketball and football coaches are paid in millions. We had a new stadium built costing $300 million which the students are still paying for even though the stadium is complete.
Unfortunately, the tuition fees are increasing every year because the universities can afford to do so as people have no option but to go to college if they want to have a chance at a reasonable job. If they don't go to college, they are stuck in low-paying jobs for the rest of their lives.
If they wanted to, they could reduce the tuition, but the universities don't want to because they can afford to get away with higher tuition rates.