If most universities in the U.S are non-profit, why are they so expensive?
I think that education in US, even in non-profit institutions, is expensive, because colleges and universities can charge basically whatever amounts they want, a practice, to a significant degree, IMHO prompted by trying to stay competitive by overspending on amenities, many of which are very remote to the nature of education and learning. Of course, there are significant costs, some of which are appropriate, such as salaries for faculty and staff, equipment for labs, some research programs and other reasonable expenses. Having said that, I believe that some of the expenses are over-inflated, such as (too) nice and fancy campuses and buildings, top administrative and executive compensation, research labs and programs that don't produce significant enough output, sports and other over-spending sources. That is not to say that tuition and fees cover all those expenses - universities traditionally rely on various financial sources in addition to tuition and fees (see the last link below). Of course, the above is just my somewhat naive (but, hopefully, not so far from reality) interpretation or, rather, impression of the complex ecosystem of higher education.

More details on the topic can be found on Wikipedia (i.e., see this section and this article) and other resources (i.e., this article). In the popular press, there is no shortage of opinions, both blaming colleges and universities for exponential increase of the cost of education as well as defending their actions and situation. Those interested in a more comprehensive economic analysis of the subject, can be referred to a significant amount of existing academic research, such as this thesis.