well, my answer is that playing those games is not so fun. When playing the game is just a chore and an obsession to increase numbers, better make the pc play alone instead of draining my precious hours on silly timesinks.
let's imagine any mmorpg as an idle game. the core of the game is just a skinner box, push the button and have more numbers more loot, more something. but it's not so fun, probably just a compulsion. something I'd easily let a pc do. A repetitive, boring task that is just...well, grind.