Historically, new parks just cannibalize attendance at existing parks. People don’t get more vacation time just because there’s a new park, so rather than add another day to see the new park they just cross one of the existing parks off their list of things to see.
Universal doesn’t mind this because they think they can steal a day that people currently spend at Disney. Disney wants to steal back the time people have started giving to Universal, but not at the expense of one of their own existing parks. When AK opened lots of guests just skipped out on EPCOT or Studios to see it.
Building a 5th park, while creatively titillating, doesn’t solve more problems than it creates, so Disney isn’t interested.