Why? Because people are ignorant to somethings that we need to explain to them. The people building look at price of all the different parts of their home, realize they want this amazing counter top for the kitchen and figure that new windows are new windows, energy efficient and think the vinyl will last, they have no idea that they get what they pay for. I had a buddies customer pay 5k for the front door and put in low quality vinyl windows, he said no amount of talking would get them to understand the few thousand dollar price increase on windows was as necessary as "their door".
I figure give it a few years and I'll get to replace them I guess.