The key point of multiple theme parks is the psycological driver that people visiting Disney World will need at least one day to visit each of the parks. Adding a fifth gate likely would not get guests to stay any longer as the current collection of four parks and two water parks (plus ample shopping) works well to keep guests on property for 5-7 days (the quintessential amount of time that most American's take off for vacation). When you control the psychological drivers that will keep most vacationers interested in what you have to offer for a full week, it becomes much easier to get those vacationers to stay on site, use your transportation, and keep them on site for the full vacation (meaning 100% of their vacation spending goes to you [Disney]).
Universal's issue now is that vacationers do not perceive them as a "full week" destination. With two theme parks, a water park, and shopping, psychologically, many people will think that there is not enough to fill a full week and will look else where for things to do to fill their vacation. When you start looking at doing Universal + SeaWorld/Kennedy Space/Busch Gardens/etc, all of a sudden it becomes much easier to rationalize staying off property and when you stay off property, the likelihood of you dining off property increases greatly. For this reason, Universal needs to add another gate to drive that psychological narrative that you can come to Universal and fill a full week without needing to go anywhere else.
Disney does not have that issue. Would adding a fifth gate increase attendance? Sure, but adding a fifth gate is complicated and messy. Adding a fifth gate you need to add all new infrastructure, update/new transportation/transportation routes, all of the four park advertising needs to be overhauled. Adding a fifth gate would also (very likely) cannibalize business from the other four parks (as happened when Animal Kingdom opened). On the other hand, you can probably add a fair number more attractions by building them at existing parks (where the infrastructure already exists) than the numbers of attractions you'd get at a new park spending the same. Makes a lot more sense to build up parks like EPCOT which have the capacity to hold a LOT of people (in the midways... not in current ride queues) than to build a new park.
