Why not?

Even at the top of the high end, products are built to hit certain price points, with certain parts and pieces decisions made accordingly. The car tire analogy is an accurate one - good enough to last the buyer a fair amount of time but not the best one available. Secondly, not every manufacturer spends time testing after market tweaks with their products. Some do but many are already busy enough just keeping their products available, especially given today's product pipeline shortages.

As a scientist I can understand that if one shows something happening on a DC supply then I know that the cord changed things.

@holmz Ah, I see.  You don’t trust your own ears.  Got it.  You and @ghdprentice would have an interesting conversation as two scientists with polar opposite views on cables.  Why not just try an aftermarket PC just to see/hear for yourself?  What are you afraid of — that your comfortable and convenient world of theories and measurements might implode if your ears tell you they’re hearing actual improvements?   Or maybe you won’t find any improvements or maybe even decreased performance.  Either way, that’s called learning — a concept I’d think a scientist would embrace.  

Why not just try an aftermarket PC just to see/hear for yourself?

@soix : it's not going to work. They will never try anything for themselves. Trust me, I have attempted to do this several times over the years. I have even offered to send them for free power cords / cables, nothing fancy, basic good quality stuff at about $100 - $200 range. They have always refused. It is a "thing" for them. They will never try out of principle. I have given up attempting to help them. I already know the story the moment I see their very first post. It is an exercise in futility, no matter what.

@soix and @holmz  Funny story about that. A friend lent me a pair of MIT Terminator2 speaker cables. I put them in my system, replacing some Crutchfield generic cables, and was amazed! Clarity, texture! I was so smitten I bought a pair of MIT Terminator2 interconnects just to see if what I was hearing could be further enhanced. Sure enough, better still. Enjoyed them for a couple of months, reveling in what I was hearing. Time came to give them back, which I really was sad to do. Determined i would have to get a set of my own. Replaced them with the old cable. Everything still sounded good. I could not find fault with the system.

Possible conclusions;

1. I am a hopeless audiophile wannabe who should be banned from any audio show room hencewith.

2. The MIT cables magically transformed my system when I installed them and somehow left their goodness there even after they were removed.

3. My expectation of how good I expected those MITs to be clouded my judgement and dimmed by auditory memory sufficiently that I could fool myself.

4. The MIT cables did change the sound and I heard that and thought it was "better", but the change back to the original was not as drastic as I had expected and I discovered I liked that "flavor" just as much as the MITs.