Well there are two types of people, those who can put a system together themselves, we will save a few Hundred bucks in exchange for 8-10 hours (asssuming all parts work) of work. Setting up the bios, putting the thermal compound on properly, hooking it all together, makeing sure all the screws are proper tight, installing the OS .... all time consuming steps well worth paying someone a few hundred to do for you.

Those who don't will pay the couple of hundred extra, and be well worth it, the guy has to make a living ... I would not begrudge him fair profit.