I think a few things come into play here. One, they want to assure the buyer that they design is optimized within the component itself. By admitting a power cord or feet make the difference, would admit a shortcoming within. Although some have added upgraded feet and called it out in the sales information. Two, I think they realize that a system synergy and environmental influences can alter the designed sound goal. Three, maybe they feel that their design can't be improved upon with PC's, fuses, feet, etc. But I really think that they focus on what is in the box and leave the rest to the consumer to experiment. And the benefits are realized by the cable, shelving, fuse, and other aftermarket manufacturers to exist. Or at least in my opinion.
Why do designers/manufacturers...?
If audiophile grade feet, power cords, and fuses can really improve SQ, why don't high end designers/manufacturers just incorporate these audiophile grade components at the time of build? Why sell an under-optimized item that can be easily improved upon by the owner or a modder?