I of course don't remember the details so I looked them up. Here's an interesting document that doesn't actually go into the fundamental question of why the engines were built in one place, partially assembled in another, then snapped together in a third:

http://www.cedengineering.com/upload/Ethics%20Challenger%20D...

The point being that if the rockets were welded and fueled near the launch pad there would have been no need for o-rings. The reason for o-rings was that cylinders full of fuel needed to be shipped around and welding them together would have meant too big a lump of explosives. Casting a big engine had nothing to do with it.