[2F/2F]

So why aren't we buying 220 backs for cheap and putting 120 film through them?

Some people do. But you can have odd frame spacings and lose frames on some cameras. When I do this on my M645's, I lose half of the last frame. Hassies lose one whole frame, I believe. My friend does it with her RB all the time; she doesn't even have any 120 backs.