It had to be, look at Dell, they're still going strong. Computers are a tough product to sell in retail outlets, the savvy people already know you get the best prices buying online, the really savvy people know how to build their own, and that pretty much leaves the retail outlets with the dumb people who will drive the store buggy with returns and silly questions.
Another poster had it right, Americans are an instant gratification society, we expect to walk into a store, and be able to walk out with what we came for. Retailing computers seems to be pretty risky, all your merchandise is like produce, you have to move it quickly, or it goes obsolete in a hurry.
Look how many small independent computer shops have opened, then folded. Sure, a lot of it is from geeks who have no idea how to guarantee customer satisfaction, but its a low-margin business at best, requiring massive amounts of handholding for people who are goofy enough to load an AOL disk into a perfectly good computer.
It may be possible to buy the parts and assemble them myself, but I usually buy computers with the latest technology for graphics intensive applications. Just because one can buy parts, doesn't mean those parts will work together correctly when assembled together. It is much easier to buy a computer from Dell. They sell lots of computers every day and can evaluate all the latest hardware and sell already assembled computers whose components work together correctly. Dell will even gurantee that their workstations will work with particular applications which is important when running applications that cost tens of thousands of dollars to buy and a significant fraction of that for annual support fees.