Hardware in games

Why new games need new hardware? Why they cant run in full details on machine 2 years old? It is still powerfull, and when was new, best games ran on it, so why new ones cant? It is pretty simple: Manufacturers want to sell their new game. They dont need poor custommers. Rich ones have good comps, poor can game buy after year, when they have enough powerfull machine. And those who have money now, they want new better graphic effects. That is reason.