Was in PC World or Currys getting a SatNav or something and some woman can't even buy the correct ink cartridge for her printer. What make you think the average joe can build a PC. Most people get scared when an electronic component is opened. Plus you forgot aspects such as:
1 - research, knowing what socket to match what motherboard. We are not born with this knowledge, it takes research and reading and constant update of knowing what is out there, what is going to come out and what is going to be obsolete. Buying an AMD CPU for an Intel motherboard is never going to work is it? But who knew? A lot of people don't even know you can't put Canon lenses on Nikon and vice versa. (lets not going into the adaptor ring debate).
2 - Even if someone online spec you the complete computer, with cables and stuff so you have every part. Little things like de-static yourself, knowing the steps to put it together is not the same for every case and every computer.
3 - Then there is the software, which is a whole other kettle of fish. Loads of people can't even keep a working, virus free machines. What make you think they can install windows?
I can map it
That's because you are a geek at heart