One word: economyTwo words. Wasted horsepower. Let's see how much BS we can cram into an OS to simply waste horsepower.
Create a new OS, put some fancy stuff in it. Ask for more horsepower and sell more hardware. It's all related to each other.
I installed the VistaUpgradeAdvisor a while back, and it was telling me I should get a new video card. I am perfectly happy with my video card, and I dont intend on changing to Vista soon.
Thats where imo is the big difference between linux and windows platforms. Linux is being developed into so many flavours that there has to be at least one version that suits your needs. Check xubuntu for example its doing the opposite of where windows is heading, minimal system requirements for maximal output.
But alot of people will go and get themselves a new video card ( or even a new pc ) just to make sure that Vista will run "smooth" on it...