Alun, the same answer applies to why they don't provide a better power supply that might not be so affected by the power cord-price point. But I must say that I don't know any designer that spends much time with power cord evaluation. Recently, I have seen a number of components that do not even come with a cheap power cord, suggesting how common it has become to leave these cords in the shipping box and to use the power cords that you prefer.
I suspect that most manufacturers think that they have properly "voiced" their components on decent power cords and that consumers will use their personal choices anyway.
I suspect that most manufacturers think that they have properly "voiced" their components on decent power cords and that consumers will use their personal choices anyway.