It's the numbers that matter in a lot of situations. Also a lot of dev time is wasted in interface development, or working on a fresh engine. Then spread that across a dozen classes, talent builds, etc, etc, vs a single player game where they can barely get one class right, let alone 34, and then balance them. On top of making the combat actually fun, engaging, and not something I need to bot in order to succeed at. It needs to be simple, while also incredibly deep. Which is why folks are still fucking around with EQ and vanilla WOW. Add simple things that exist today like better graphics, a clean interface, and a reward structure that isn't as convoluted and fucked up as a mobile game.