WIS has and will merge two worlds when the numbers work and both agree to it. Taking 4 worlds and merging them into 3, or whatever, is messy. Not from a coding perspective, necessarily, but from a user perspective. All of a sudden, you go from a world with a ton of history and a core group of users and now you've got the remnents of a bunch of worlds, etc. I believe at least one time they merged 2 worlds, the next offseason that merged world had 8+ openings. Many worlds are private and have their own rules, etc.
This happens every fall and people think the world is going to end, the worlds eventually fill, and in a few months when demand pops back up after football and as baseball starts up, things are fine.