Because they're the status quo. McDonald's is largely garbage food. You have some standouts in their menu, but largely, you know you're getting the dregs of fast food there. But they are so accessable and well known, they still remain among the top dogs out there in fast food.
Same thing with WoW. There is a lot of good content there, Legion is actually a really good expansion, but all the accumulated garbage from over-homoginization and outdated mechanics just drives me away. But it is so accessable that you really can just download and play current content and have a pretty good grasp on what to do. It's the McDonald's of MMOs.
