I wonder if the folks at Disney would be surprised by this thread. There are a lot of perks, like EMH, transportation, etc., but it seems the overriding theme is that people love the feeling of being removed from the "real world." Count me in that group.
I lived in Orlando for a while, and visited WDW quite a few times. I moved away, and my wife and I visited some more, but stayed off property. Finally, once we had kids, she talked me into staying at a Disney resort, even though I thought it was a waste of money. I would never go back off property now. The feeling of being in a whole different world is just the greatest, to me. The only thing that compares to it is staying at an all-inclusive resort, but even that isn't the same. Even at an all-inclusive, you feel compelled to go out and explore the area, but at WDW, you just want to take it all in.
As someone else said, it's like being transported back to childhood, and nothing interrupts that feeling for the whole trip if you're on property. I know the food will be good, the rides will work, and everything will be clean. I don't even mind the sometimes lengthy and crowded bus rides, because I don't have to think about where we're going. In short, I guess it's because I can let someone else (Disney) do the worrying, and I can just be along for the ride.