Not sure why you’re downvoted, but I think you’re close to at least partial reason. Having one device last a few years makes it easier to replace under warranty, if what you’re truly trying to do is provide superior customer service over competition (they probably have lost of spare parts, as it makes sense to order bulk and of extras, unless you’re super greedy company who has no faith in its product).
And as I assume you’re suggesting, they’re going to make their money on the games that people buy to play on that device, so why waste effort on making miniscule improvements that may end up hurting your brand and labelling you a money grabber.
Don’t get me wrong, Valve know how to make lots of money, but I bet they’re smart enough to try and do it in a “sustainable” way from their perspective.