The why is, of course, as others have said, that people are willing to pay it. For me, the why is, why? Don't get me wrong, the Poly holds a lot of good childhood memories and I love it there. With that said, it's still just a room, at a nice hotel. I go to Walt Disney World for the parks. For what the Poly costs me for a week, I can stay two weeks at another resort. Let's see; a week at the Poly, or two weeks in Walt Disney World at other very nice resorts? I'll take the extended stay thanks.
All that aside though, Disney room prices in general are what are going to start pushing our family to look elsewhere to spend our vacation dollars. When $3-$400 a night is becoming the norm even at "lower end deluxes" like AKL and WL, something is wrong. For that kind of money I can stay in 1-2 bedroom accommodations at many different vacation destinations around the country. Heck, for the price of a "value" resort at Disney, I can stay at a Hampton Inn, get two queen beds, a VERY large room, and a free hot breakfast!