It's called "price point"
One reason manufacturers don't include upgrade parts [including power cords] is due to maintaining a certain price point. Let me give an example. I worked closely with the manufacturer - basically a one man company - of my amplifier concerning upgrades that would have a cost effective impact on sonic performance. We decided on four modifications on his "production amp". All but one of those were reasonably priced with the total cost of parts [alone] coming to about $150.
After the amp was assembled he spent the next 3-4 days burning the amp in and compared the stock amp to mine with the mods. In his words he felt "the upgrades did improve the overall performance" BUT that he would not consider those upgrades into production because of A> Added cost and B> Value. He felt that while the improvements were evident, the results were sublte and outside the price point of his product. But seeing it was my money, we both felt the project was a success [he performed the mods at no additional cost, I just supplied him the parts].
Which brings up another issue. We chose [or I chose and then sought his opinion/approval] the parts to address specific replay characteristics I was hoping to enhance. Obviously, the parts he chose for the stock amp did not take this into consideration, instead were chosen for other reasons. This would also apply to power cords. In fact, one of the production changes he made in his amps was a IEC receptical. I asked him why and he said, "because my customers requested it". This way they could experiment and use a cord of choice rather than be "stuck" with a hard wired cord.
So reducing cost and individual attention to customer needs are but two reasons that such "upgrades" are not included in stock units. As far as compromising performance. Due to maintaining price points this is a delimma but even with a manufacturer's TOTL component they would admit that even greater performance could be attained with "better parts" or supporting devices. And of course, there would be no way they could build each amp specific to each customer's need. That burden, if they choose to accept it, falls on the individual. Look at tube components, do they come with exotic NOS tube varieties? No, they usually come with generic tubes that most people substitute for their personal favorite. To deduce that a product is altered because it's suspect is dubious. No, it may be adequete, but some people will always strive for the extraordinary.
MikE
I also find the, er, um, esoteric tweaks pretty darn funny.:nutz: 
