There are three Demo Day events, each composed of two identical sessions. The only reason we have multiple events is that we don't have enough room for all the investors at once. Though it does have the additional benefit that if someone screws up their presentation, they get other chances.
You can't transform an auditorium into a reception in place, which is what we do to the orange room at YC after the presentations. After the last talk the founders pour in and we scurry about folding up all the chairs, and about 3-4 minutes later the room looks like a (big, loud) cocktail party instead of an auditorium.
Most hotel conference rooms/ballrooms are set up with portable chairs and I don't see why you couldn't do the same thing there, but something seems ineffably worse about that scenario to me. Maybe that with everyone presenting at once to all of the investors, there will end up being fewer total conversations between founders and investors.
