The dorms have shared bathrooms, often shared rooms with doubles, triples, and quads, and shared living spaces (lounges, study rooms, laundry, etc.) in addition to the dining halls. Moreover, in our dorms, we have effectively no way to eat if the dining halls close down. Since mine has a dining hall for us to eat, it has only one kitchen for the entire dorm, which is ~500 people.
Edit: It's also worth pointing out that MIT students are at very low risk right now. Sending us all home before we're at a medium/high risk level is the right move, so students aren't bringing it home with them. Nobody on campus has tested positive yet (as far as we know), and once it reaches the campus it would spread like crazy. Better to preempt it and send us away now.