Lots of research with animals and humans analyzing choice has found that changing from one option to another (e.g., the decision to move from one game zone to another) is controlled by the relative rate of reinforcement for those choices AND the effort required to change from one choice to the other. For example, studies have manipulated the physical distance between two keys in an operant chamber and found that as the distance increases, animals tend to respond more exclusively on the key with the higher-rate of reinforcement. As the cost or effort to change decreases, choice becomes less sensitive to differences in the rate of reinforcement among the alternatives. Instead of manipulating physical distance, experimenters have also required subjects to respond on a change-over key. Increases in the response-requirement on the change-over key produce effects similar to increasing physical distance.
So, increasing the effort to move from one zone to another likely produces real changes in how players distribute their time in those zones. Good game designers probably distribute the best in-game rewards as an increasing function of the difficulty to obtain those rewards. It's probably also similar to having NPCs of varying difficulty. So, the question could have been, why don't MMOs just equip players with the best stuff when they first start?