I think the biggest reason, one you discarded, is the fact that most of Las Vegas is paid for by gambling profits. Notice there aren't any big resort-hotels in Vegas without a casino!
There are plenty of rooms in Vegas that cost more than those you mentioned. And the rates you mentioned are discounted rates, not "rackk" rates. Try getting those kind of rates during a heavyweight fight weekend, or a major convention. Good Luck!
I have stayed at Disney Resorts for $49.00, and for $79.00. The discounts are out there, if you can find them , adn if you travel during off-peak times. Why should they sell a room at a discount when people are more than willing to pay full price? If you want cheap, stay offsite; there's plenty of competition for your dollars there!
It's my opinion that Disney resorts are a great value, even at full rack prices. When I can find a discount, they're even better.
Your opinion may be different.