Why are colleges so expensive?

When I went to pay my school bill it was crazy I have never seen a school bill so high. I had to pay a lot and since I am a freshman I am not use to bills that high. The hard thing is my family is not that rich like most of the students here and it is hard to come up with that amount of money is such a quick time. Since I am on the work study program it pays for part of my schooling but it doesn't pay for all of it and I just don't know what to do. There are seven people in my family and it is hard to pay for a family that is that big.