They are, because their state funding is increasing at a lower rate than inflation, and because they have to pay for new buildings and equipment to support growing student populations.
And contrary to right-wing pundits, growing student populations are not a problem, they are the stated goal of public universities. You can't just raise tuition to discourage people from applying because the entire point is to get as many applicants as possible.
Only at for-profit schools, which are currently a small fraction of all universities.