The area at the top of the cabin is pretty much 'dead space' in terms of the airframe. In contrast, there's a metric shit-ton of wiring and various piping under

The area at the top of the cabin is pretty much 'dead space' in terms of the airframe. In contrast, there's a metric shit-ton of wiring and various piping under the floor (with easy maintenance access by popping up floor panels). There's also other important things under there, like the wing spar pass-through :) If you raised the seats to put the luggage there, the curved sides of the cabin wouldn't allow you to put a seat near the window, so you'd lose a lot of valuable seating area (which airlines track to fractions of an inch)...