When an Aircraft is designed, I believe that aesthetics might be an impoertant aspect. These huge things must also wear a the airline's livery. Remember when the 757/767 was stlill on the drawing board (7N7 at the time), remember what rhey looked like? I'm sure Boeing that when Boeing proposed a design they would have designed something just as efficient, if not more as what finally rolled off the factory floor. It was when this was proposed to most airlines I guess they liked the numbers, and not so much how they looked. I belive the same thing happened with the 777, I remember seeing a drawing of something like a 767 with some kind of an upper deck
I belive what also must be considred is that airplane companies usually try to offer some kind of continuity in design; a family of models which they would also want to produce at minimum penalties in manufacturing processes- Desiging something too radically different would probbly require too much investment in manufacturing. As we all know, airlines benefeit from operating a family of aircraft be it from an engineering standpoint or from a crewing one.