Conveniente - Penny Arcade:

That's because even if you are playing all by yourself, alone, there is no single player mode because Diablo isn't a single player game.

One of the reasons I continue to love Warframe is they have this thought out - If you’re in a story mission, or any game mode set to Solo for grouping, you can actually, honest-to-goodness, pause by opening the menu. As a grown up with things going on with kids, cooking, etc. this is an affordance I use quite often and I bless the devs at DE for it.