There are a couple of scenarios where you can possibly build cheaper. Consider when the purchaser is looking for certain components or to meet a particular specification. In those situations they would have to pay someone to build a custom system or pay the vendor to customize the system (in those cases the vendor frequently charges a premium).

Of course the big question is, why aren't household devices/appliances using standardized and interchangeable components? In many cases, it could probably be done. I suspect the main differentiator is consumer expectation. A significant number of consumers of bikes and desktop computers will repair, upgrade, and accessorize their purchase. Very few people will expect to do the same with their blender or television.