How about a real simple answer to the location of spoilers on an aircraft...

What is in the wing??? FUEL. The main load bearing area of the wing in 99.9% of aircraft is a big fuel tank... why would you bother building anything in that area when you can put them at the back where there is easy access. Placing the spoilers over the tank would require plumbing hydraulics, wiring, and control cables through the fuel tank, around it, or etc... not very practical now, is it?