First of all, tuition at many public colleges and universities is about $5000-$6000 per year. You only get up to those higher figures if you are going to a private school or are adding in room and board and textbooks.
There are a couple of reasons why it is so expensive. The biggest reason is because it costs money to provide education. One of the largest chunks of a college’s budget is health insurance and benefits for its employees. As you know, in the US, employers are responsible for a part of the insurance and insurance costs in the US are very high. Also, many US colleges have dorms and students live on campus. This adds to the cost. I would also point out that colleges and universities are not highly-subsidized in the US. While they do receive some state and local funding, they also rely on tuition for a part of their revenue. We tend to like to offer financial aid on a per student basis rather than subsidizing the whole college.