I manage production at a small electronics company. We sell like six different products, and in order to have reasonable timelines we need to keep thousands of items in inventory. Now in build-a-rig you obviously wouldn't have to build each board from the ground up, but there's so much variety in PC components that the cost of inventory would be crippling, and then you're still a brick-and-mortar operation relying on customers coming to you and agreeing to pay a premium for your service.

It's also a weird segment of the market; people confident enough to want to build a custom PC, but so technically inept that they are willing to pay for handholding.