As a company, you want the maximum amount of profit for the minimum amount of investment and expenditure. In the case of WoW, you want to provide as little WoW as possible while maintaining the maximum amount of profit.
There is, in essence, a Goldie Locks zone where you max out the profit you reap while minimizing the development time and effort, resulting in the highest possible return on investment. Basically, Activision-Blizzard does everything it can to push towards the minimum investment while maintaining a certain level of profitability.
The more gametime they can squeeze out of every drop of content, the more “profitable” that content and thus their investment is. They’re absolutely desperate to make sure you never “run out of content” and cheapest way to do that is to make that content take longer.
Why would Ihop add sawdust to their pancakes? Because it’s cheaper than flour. Why would Activision add grind to their World Quests? Because it’s cheaper and easier than novel development.