It seems like the 767 will be in production for a few years longer then the 757. My question is, why did Boeing decide to produce the the two planes separately? They were designed together and have a great deal of commonality with each other. However Boeing began 757 production in Renton and 767 in Everett which is over 30 miles away.

It seems like they could have saved costs by having the planes produced side by side. the 757 and 737 aren't very similar with the exception of being narrowbodies, and likewise the 767 and 747 don't share much in common either. There have been a number of threads about Boeing production efficiency recently and how it isn't as good as Airbus. So why did Boeing put themselves at a disadvantage from the beginning by eliminating the economies of scale of an integrated production system? Was it due to land space, unions, or some other reason? If they were produced side by side, then would the 757 probably still be in production rather then getting phased out because of poor sales and in favor of increasing 737 production?

Any comments are appreciated.