There are only two ways to give everyone a private dorm room:
1. Drastically reduce admissions until a 1:1 ratio of rooms to students is achieved. Once rooms are full, you have to reject students. Overall college admissions is cut significantly.
2. Massively increase tuitions so that more single-occupancy rooms can be built
So which one do you choose? I suspect most of these proposals rely on a world where money is infinite and nothing has to be sacrificed to get there.
Umm.. these elite universities are sitting on tens of billions of dollars in endowment funds. They can easily build dorm rooms if they care to - especially given the universities already own the land and have vast amounts of unused land.