Completely agreed, but for the average person it is a lot of work to disassemble a laptop. Tools required, thermal compound, researching the procedure, etc all add up to hours of labor and/or costs. While tech savvy people have all this laying around and generally know how to manipulate all the little connectors, most people don't. This is why repair shops can get away with charging as much as they do.
Going from a dual core to a quad core in the same series has its benefits. But it is often cheaper, and much less time consuming, to just buy replacement hardware. The other system can always be sold, donated, or given as a gift to a family member.
Replacing just a broken touchpad button on a Dell XPS required me to separate all the wiring, the motherboard, the screen, and about 30 screws. It was vastly easier to replace the CPU fan assembly in that situation.