Because, once designed, the glowing reviews and purple prose typically applied to such things define them as. . . . complete. And then, the human ear yet again, evolves beyond that?
Above what was known 10 years ago, what has really significantly improved since? My current favorite description revolves around tweeters, with a penchant for. . . ."sparkle." Well, tweeters have been able to portray accurate and audible, tweetery sensations for decades. For one, we heard it sound exactly what we were hearing live, and for another, we have been told by the manufacturers that they have, through each and every revision of that technology, perfected it. Or is it more a case of, "Forget all we have said, we have it figured out. . . 'this' time."?
Sorry but, technology, at least with regard to music anyway, has had to have surpassed the ability of the instruments actually being played by now.
Designs should not become outdated by now or even from 10 years ago. The most they should become is mechanically worn out from heavy, continuous use. A decade is a good mark for service expectations, unless of course you happen to spend $10k on it. Then, it should be at least continuously serviceable and future proof for the rest of one's life. At least if we are to take verbatim, what we are being told.