Why does Boeing build airplanes in two factories instead of in one factory?
Building airplanes takes a lot of real estate. Lots of parts, many of them very large.
Boeing started with a fairly good size factory next to Boeing Field in south Seattle. During WWII, they opened the Renton plant to make B-29s, then 707s, then 737s.
For the 747 they built a new plant in Everett, 30 miles north of Seattle. It's the largest building in the world by volume and now makes the 747, 777 and 787. Even then, they had to open another plant to make the wing spars down in Frederickson, about 40 miles south of Seattle.
As a side note, that means we get to see the unique dual-driver trucks that transports the wing spars up to Everett. Here is a video:
https://www.youtube.com/watch?v=Rn0OaxD3XuM Pretty cool, eh? Not something you'll see anywhere else.
Witchita made B-29s, then B-47s, then B-52s. It was likely more efficient to use Witchita to build the 737 fuselages than try to find space locally. The area around Renton was all built up and so there was no way to buy up hundreds of acres to expand the old plant.
Todd