Because not everyone can afford to just upgrade to the next device....?
This, incidentally, is almost certainly the purchasing behavior of the vast majority of computer owners. RAM and disk upgrades (particularly on laptops) are performed by a fairly tiny minority of consumers. Yet it seems disproportionately cited when shopping, likely due to an illusion of control or the laughable (in computing terms) myth of "future-proofing".
Also, I do upgrade every N devices. I'm not someone that buys the new model everytime they release one. I guess I should have phrased it "because not everyone can afford to upgrade to a new device".