Multiplayer isn't required for those choices to exist, but the presence of multiplayer does enforce that standard, and removes the temptation to cut corners in particular situations to make certain quest scripting easier, or if time starts to run out, etc.
Rather than try to think of things that would prevent these options in a single player game, try coming up with other motivations that would get a developer to decide to more than quadruple a lot of the dialogue and add many more quest solutions based on different party members. Freedom and immersion, etc, could be done quite well with much less work. Of course a particularly strong vision or gameplay plot mechanic for a game could do this, but the fact that party based single player games do not generally have dialogue and quests based on all the different party member races (if an option) or origins (if they have any) means this is not generally considered a desirable feature given the relative benefit and resource cost compared to other aspects of the game.
