That is the point ...
Now, when they create characters (and no, its not just about Origin companions, its about all NPC) ... all they need to remember is that character can be from BG / Underdark / Astral Plane (thats it, isnt it?) and if so, add apropriate reactions ...
If surface dweller would have 4 cities to pick ... the number of potential reactions, would be more than doubled ...
Now count all the others permutations that can be created and you find out that with adding single another option to react, the amount of work is incerasing exponentialy. :-/
Example:
I heard few years ago that some people are trying to create new generation of procesor, that will be able to recognize 3 states, instead of 2 ... so no more 0 and 1 ... but 0, 1 and 2! ...
It may not seem like much at first, but with single bit you were able to create only 2 states (0/1) ... now you would be able to create 3 (0/1/2)
With two bits you are able to create 4 (00/01/10/11) ... now you would be able to create 9 (00/01/02/10/11/12/20/21/22) ... so you see that your calculation power was allready more than doubled!
With three bits ... im too lazy to count, and i believe you get the idea.
I dont say its exactly the same, but some paralel can be found even here ...
Sure, we could add "unimportant" choosing the citty of origin, if noone ever would react on it ... but many people would not exactly be happy about it.
And once we start to concidering that "just one another choice" ... we might be easily surprised how many additional work we just created.
Therefore its just easier to pick some place, where we all are from ...
And if we are not, let us to make some little conversation in our head.
