Wouldn’t they be in the best position to test or design the optimum cable for their component?
Yes and no. They could pick the best PC that works IN THEIR SYSTEM, but that’s in no way a guarantee that it would synergize best with a customer’s specific needs. Given how cables can sound different in different systems and the quality of power, power conditioners, etc. varies greatly in every system there’s really no “one size fits all” solution for cables and why 95% of all manufacturers don’t include PCs or interconnects with their equipment. If the cable qualities were consistently translatable they’d include or at least offer them. Making cables optional is a thought, especially if they can be offered at a discount, but most seasoned audiophiles won’t do that as they know what cables work best in their systems and will in all likelihood stick with those. Obviously including any quality cables would jack the price up of any component dramatically after retail markups and make them look much less competitive compared to those that just provide a passable cord and let the customer decide where to go from there. If it made sense to offer better cables with their gear manufacturers would be doing it, but they clearly know it’s to their disadvantage to do so unless they’re in the cost-no-object category where they can bury the cables in the astronomical cost of the equipment.