Partly because dorms will kick you out over longer breaks (winter and summer) and will expect you to leave for shorter breaks (thanksgiving, if you’re somewhere with a weeklong break and spring break).

Over winter break I had to turn in my room key to the desk and my access card was deactivated. Those who paid to stay thru winter had to move to a specified dorm.

So yeah, you still have to depend on someone else for housing while in the dorms, at least how I experienced it.