Here you see Moore's Law at work. A new generation GPU is targeted to have about the same price as the previous generation. The specs of the new generation are increased/improved until that target price point is achieved.
Someone mentioned Moore’s Law. Secondly, there’s no incentive to upgrade if it’s not competitive to do so. So a new, cheaper, faster electronic is a better sale. Third, used electronics lose efficacy over time. And the way software is structured nowadays, we need hardware to keep up with more functionality more than ever. Before, software used to be tailored to the piece of hardware. These days, all software and hardware must be somewhat plug and play compatible to an extent.