Because you and just about everyone else tossing out that tidbit of "advice" on the forums are doing so despite being fully aware that most people willing to criticize the game still have a vested interest in it. In other words, it's an exceedingly disingenuous way of trying to silence criticism from behind the veil of, "well, it's okay to quit if you're not having fun," regardless of whether or not people still have aspects of the game they enjoy. Which, fairly obviously they do if they're still subbed. You can play a product with a significant number of components you no longer enjoy and still find some joy in it.
Some people don't like the direction the game is going, and those people probably will quit once it reaches a point where the things they still enjoy have been taken away from them. Simple as that.