Just about any person with an IQ over 80 and some remote enthusiasm to learn can put together their own computer. It's just that it's easier to have somebody else do it, and with todays competitive prices, it's almost cheaper to have somebody else do it, and then lay the burdon of support on them, instead of yourself.
About the only really difficult thing anymore is installing the OS and loading all drivers. But, with Microsoft putting almost all OS's out on bootable CD's anymore, that's really not an issue either.
It just comes back to laziness and the lack of desire to actually read a manual or "how to" guide. As for picking out parts and what not, with the abundance of "hardware" sites, all you need to do is sit down and do 20 minutes of research looking at their system guides. Pick out how much you want to spend and just go to newegg and order the parts that the site had listed.
But again, this takes time, and a little innitative, something that a lot of people don't have, so they just call dell and say "Give me the best thing you have for $XXXX". 3 days later they have a computer sitting on their doorstep.
As for things that take a little more "know how", 3D drivers and direct problems can be a bitch to sort out. Sometimes troubleshooting network settings can be a royal PITA too. But really though, just about anything you encounter can be solved with a little bit enthusiasm and some time spent scouring tech sites and trouble shooting BBS's. It just takes time and energy, once again, something that many people lack.
I don't think there really is much skill involved in building a machine, and in fact I tell people that regularly. It's just a comfort thing, and requires a few minutes of reading to figure out. I tell people that they could just as easily do it, but to them, it's just easier to pay me $30 to swap out a card, or install some software.
It's their money, and I'm not complaining about getting paid.
It's really no different than me changing the oil in my car. Yeh I can do it, but it would require me to go to the store and buy oil, and an oil filter. Then I'd have to go back home and put my car up on the ramps and crawl underneath and do all the dirty work myself. Sure I can do it, and I'll save maybe $5 or $10. But, it's half an hour of my time and if I don't screw the oil filter back on all the way or something stupid like that, it's my fault then. I'd just as well take it to a dealer and have them take 20 minutes and change my oil for $17. I get free pop, and a chance to read some automags that I otherwise wouldn't be able to read. That's worth a couple extra bucks to me. Just like it's worth it to someone else to spend a couple extra bucks on a Dell or something along those lines.