Because I want to be able to make an open game in the hopes of finding a new friend to play with, without the possibility someone is going to join my game JUST to grief, camp, and steal boss kills. Blizzard said it perfect, the only people who actually enjoyed this option were griefers.
Adding the option when you create the game won't work, because I highly doubt almost anyone would want to make a game open just so someone can come and try to kill them while they are farming. Want pvp?, that's why we have arena.
Open world PvP doesn't work as well in Diablo as it does in an MMO with a large, continuous world. The progression through the world is fairly linear in Diablo, so there are not really 'chance encounters' to PvP. There's also not really anything to fight over, especially with everyone getting their own drops.
I'm happy with the PvP options in D3. Maybe a couple more formats in the future, but I don't see a lack of hostility as a drawback at all.