Disney's California Adventure detracts from long stays at the Disneyland Resort. Walt Disney World has many more offerings than Disneyland Resort. There is also a lack of non-park competition in Florida (exactly what DCA tried to replicate and keep people away from).
Magic Kingdom is also "Disneyworld park". There are people who will go to Florida, hit the Magic Kingdom and be satisfied that they have been to "Disneyworld". Completely oblivious that they passed a sign saying "Welcome to the Walt Disney World" miles back and passed a lot of stuff on the way.