I guess it might be because you could cheat in timed games.
What do you mean by that (cheat in timed games)? They need a pause button for all their games. Look at the Club Penguin app (they have a pause feature for the games). Why not on desktop?! If we need to go the bathroom, get a glass of water, or anything that happens in reality, you can always click on the pause button! Then, we resume on the game. This is a reason why we need it on the games. Sorry for the long comment, I was proving my point.