It's because the game is kind of always online. The way multiplayer works is that you can leave messages in your world and other players will see them, and you can also get invaded by other players if you open your game up to it (usually it's also a power up so you can be tempted to do this even when you don't necessarily want to fight other players). If you could pause, it would make things weird with the multiplayer because it's supposed to feel like a fluid real time world.