Why do we pay tuition for 3rd and 4th years?

I'm honestly just curious as to why there is tuition for 3rd and 4th years of medical school. I'm sure there are administration costs, but essentially we are providing work in the hospital. Residents and attendings don't get paid extra to teach specifically while they're doctoring on the floor do they? Can anyone provide some insight as to how schools get away with this.

Why do we pay tuition for 3rd and 4th years?