because you don't have to be concious at all to play mge. the idea for tf2 for me at least (idk about the others) is just to kill time or in between reading books to refresh my mind because i just zone out and i dont focus on anything and before you know it 4 hours have passed and then i can go skate with my friends
i play CA quake live too for same reasons and its a lot funnier because the quake mains think im a hacker but also gotta kinda use your brain for it and i cant be good at it with 500 ping so i dont do it as much as pub/mge