I had that conversation with a manufacturer in the mid 90's when aftermarket power cords where gaining popularity. He said, the main reasons are cost and results. We talked about cords making a difference and he agreed that they did, however trying to decide on a cable that would work for all systems is impossible and if they upgraded the cord, they would have to pass the cost on to the buyer, who might not like it and replace it anyway. They included a decent molded, 14ga cord with the units and the customer can upgrade or not. You might also note that at shows, many exhibitors are not using stock cords on their gear.
