Why not?

Audio Note for one.

Thanks for contributing that knowledge @alexatpos !

If the situation is as you describe @nonoise  then it really is impossible to talk about matching cables to equipment at all. It puts us all adrift in our own little bubbles of perception with no common points on which to moor. If you will excuse me it is a bit reminiscent of the so called Dark Ages, prior to the Enlightenment when the all purpose answer to every question was "...because God made it that way". Is it not?

PS Audio recommends specific cords and cables for their products...I have read, and been told by manufacturers, they include a good power cord that "non believers" will be happy with, and assume others will prefer to select their own cord, rather than have an expensive one included...Classe used to recommend an upgrade cord over the included cord...it's interesting to see how few use the included cord at audio shows or in their in house listening rooms...

@bruce19 Forgot to write that once I had Krell Evo 302 amp and got the power cord with it as well. It was 20A thick wire and it almost looked like ordinary 'black' cable that you get with other gear (only much thicker and surface was smooth, like insulation on Cardas cables) but it was 'better' sounding than few that I tryed on its place. Could I perhaps find a 'better' one? Perhaps, but, imho, it takes lots of time and lots of cables to find the one you consider the 'best' (even if you have the idea what 'type' of the cable or the 'sound' you are looking for).So, sometimes if some piece of gear plays great already with its power cord (like that Krelll, finding the aftermarket cord is something that I worry less about) Imho, the power cord on any component must be in synergy not only with that piece of gear, but with the rest of the system, including all wires.For example, sometimes is possible to find 'better' cable (per se) for some component, but some other might be better suiting for the sound of the whole system

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.