I agree that FO games are great single player games, but the above is just not true. In every Fallout there are multiple companions that may join the player. It is conceivable that those companions could be played by a living person instead of the AI. If implemented well, FO would make a great co-op.
The story is that Companions were a late development hack (and this contributes to the bad AI and the lack of item trading support ~you have to steal from them or they charge you).
Also, NPC's are not other players, and this has a big effect on how the human player experiences (and / or treats) the game.
I've heard that (ironically) the same may be true for Fallout 3, in that companions were not planned, and were a late development hack. (though I can't point to a reference anymore ~cuz I don't recall it.)
***The only Multplayer Fallout that seems appropriate [to me], would be strict PvP where each player can hire/collect their own 'tag-along' band of NPC's; and the open world allows the players independent exploration on different (or the same) maps. Players should be able to loot (and booby trap!) locations that the other players perhaps haven't found yet, or lay ambush; or become tenuous allies that have the option (based on stats and wealth) of bribing away the other player's greedy or discontented NPC's. Players should be able to take opposing quests (one accepts the role of bodyguard, the other accepts the role of assassin) ...
But its still not something I'd want in a single player series.