Because they don't want to. Its that simple. I bet it would be extremely easy for our small indie studio to create options but you know what's easier than working? Not working so don't expect anything to change. Either go find people or just be okay with missing out on stuff. I came to terms with this back in D1 and I'm still enjoying and having fun with the game.
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Or maybe because it's not really meant to be a solo game. Why designate time to something that you don't want you're game to be?