Assembled pcs are going to be cheaper than pre-made pcs because you are doing some of the work and therefore reducing costs. It is only the same as buying flatpack furniture. You are constructing the piece at home so the cost of this being done in a factory is saved. You are also foregoing the option of choosing your own components.
Intel PC bundles a pre-defined combination of CPU, Motherboard, RAM and Cooler based on best-selling components. Each bundle is pre-assembled and has its BIOS updated and tested. Perhaps the perfect starting point for a new computer.
Assembled PC's are cheaper because a lot of the time people are going to use second hand or discounted equipment and parts in order to make their computer. They can find second hand shells and parts to make a computer online. These could be parts that came from a major computer and the person is just customizing usage of the parts to make their own device. A pre-made laptop is designed and developed by a major functioning company. They are paying all kinds of subcontractors in order to get all the parts that they need to make their laptops. This is the reason that a brand new pre-made laptop computer always has a higher than normal cost. Assembled PC's arethe do it yourself project of the computer and electronics industry. DIY'ers are going to always try and find the most cost friendly parts to do their projects. The same thing goes for people who trying to assemble and build their own computers. They do it so they don't have to pay the higher price for a name brand and pre-made laptop or any other device.