You realize the other characters in your party are supposed to represent other players, yes?
It's more complicated.
First, that's not even true in most cases. There's no way in hell that the characters in Mass Effect or Mars : Wars Log are meant to represent characters played by different players. It's one PC, his quest, his moods, and AI characters which are meant to interact with him. Romanceable companions are the worst, there's a reason why "many" people have problems with Anna and Fall-From-Grace but not with Deinonarra, and relationship interactions are the worst too, but that does not mean other interactions are good representations between the players. The intention of the devs was never to simulate a party of players in those games, not more than in a modern Final Fantasy game. Now, if you'd tell me the approach of Fallout or Arcanum is a bit different, yes it is, something about the party being a little less centered around the only PC in some ways. Only it's some ways, it's hard to deny that the plot is the resolution of the PC's very own problems, you don't gather the party at a tavern in the beginning of the game but rather have to work to get some characters inside the party. Only the player and his character solves quests and take decisions, at most companions help with solving the quests the player chooses to solve and influence the decisions the player's character eventually takes, it is still far from multiple players playing the game.
Secondly, it's fun to chat about decisions with other players, it's fun to play with other players taking their own decisions, and I mean even watching them play while your character is inconscient is fun. There's absolutely no fun to me in watching the computer playing against itself. Once again, I'm the only player, so I want to take all the decisions from the players' side, not relegating decisions to characters not controlled by players.
Note that I don't know how I would feel about playing the mess which would be Divinity : Original Sin 2 with three other characters controlled by an AI instead of three other players. It would be absolutely nothing like solo Baldur's Gate or Fallout, they'd solve quest, take decisions themselves after or not after asking for your opinion, gain some combat alone and join you for others, and, well it's D:OS2 we're talking about which is a bit unique but more generally what I have in mind is really not a competition, I mean the party is working through a same journey and in some way towards a same goal but not always altogether. I'm really not saying it would accurately represents a table of players playing a tabletop RPG either, where the party is generally altogether, take most decisions altogether, but in some way it would represent other players playing the game. Maybe there's some game which's already doing that, by the way, but I am not aware. I probably would not like it much, to me CRPGs is the genre where the player controls a party he creates at the beginning of the game, and my party and AI-controlled competing parties is the best but it could make for a different video game experiment.
Exactly what are you roleplaying as when you are the entire party?
It's an oriented and complex question. I mean when it comes to video game "playing as" is really not particularly a CRPG thing. Now when it comes to tabletop, how important the "playing as" part is depends on the players. For many players the characterization of the character and what comes with it especially in the theatrical part (as opposed to the dice rolling one) is the most important part of tabletop RPGs, and for more than some of them also in CRPGs. There are often some limits, in Live Action RPG I don't know, never did that, but during tabletop my sessions were never about the elf saying some elf thing, the dwarf saying some dwarf thing, etc... whenever something happens. Then, to me, it's not that important. I don't think too much about it, it's more that if I'm playing as the guy who's good at lockpicking then I'll pick lock. Nothing which really requires you to "play as" one character, so clicking the character who's good at lockpicking to pick the lock and clicking on the mage to cast fireballs is all I need when it comes to simulating role playing. Note, finally, that I've not played Storm Of Zehir where it's supposed to originally come from, but I've played The Hearkenwold and I really like reading the dialogs specific to the different characters in my party and pick the coolest one.