So what's the advantage of building one? Am I missing something?
There really isn't much (if any) cost savings building your own machine. The only way it works out is if you're trying to do something specialized and it requires you to either buy too many features you don't need, or pick from a high end line from the manufacturer.
What you do get when you build from parts is the ability to have exactly what you want, and have a machine made from standard parts, not the fully integrated system you'll get from a big name maker. The problem with the latter, that other than adding RAM and swapping a hard drive there's not much you can do with mass market PCs. They usually only have room for two HDs, and one optical drive. Their power supplies are often custom, and can't be replaced except with an identical part. Sometimes even if you don't order the maximum RAM the machine can be configured with, they'll still fill all the slots with smaller modules. So while technically you can upgrade the RAM, you have to throw out all that you already have. You also get the satisfaction of doing it yourself.
That said, it is ofter still easier to get a pre-built machine, use it until the warranty expires and then buy a new one. Even ones built from parts can only be upgraded so far before the new parts aren't compatible with the motherboard any longer.
