djones that is just a silly argument. Take for example an automobile Ford could make tires for their cars themselves it isn't practical, Pirelli has invested millions of dollars perfecting their designs, molds, tread patterns, carbon black compounds, etc.
You are assuming that a power cord is a simple piece of wire, sometimes there is a lot more going on with weaving, custom materials, custom dielectrics and so on.
Cable companies specialize in creating vey advanced designs that are sold because they a product sound better even if sold wholesale that additional cost will have to be factored into the products selling price which now will be more expensive.
Dave and Troy
Audio Doctor NJ