For many people the ultimate game experience would be a vast imaginary world where you can do anything you want. Total freedom as it were. And that would certainly be something. But take your personal top five games of all time and ask yourself how much of that freedom they actually contained. Also, ask yourself if that was the quality that made them fun.
Game developers like to brag about how you can destroy or interact with anything in the environment, but while this is a nice feature it is hardly what makes the game great. At first you may marvel at the possibility of being able to completely eliminate an entire building or pull every lever at an abandoned construction site, but after about half an hour you get annoyed that your pulling the levers actually has no effect whatsoever on the game.
Freedom is great, but it has to be balanced with an equal ammount of content in order for people to actually appreciate it. Smashing pots in Zelda is fun for a while, but once you've maxed out the rupees in your wallet you're not likely to do it anymore. It has no additional effect on the game. Jumping off a ramp and go crashing through a window before landing on a grinding ledge in Tony Hawk can be exhilarating but what actually makes your pulse go up is the fact that it gives you a point multiplier.
Players love the illusion of freedom in, say World of Warcraft or GTA, but without a goal and limitations the game just simply wouldn't be fun.