What if the reason is as simple as quality?

There wil be a total of...what, 7 companions? Fully voiced, animated companions. There has to be a limit to those, I get that.

And it would not be fun to get the same ones on every run. Nor it would be okay for us to run around with mute mercenaries adding absolutely nothing to the story.

Seems like a design choice made quite consciously and does not ruin anything as many here imply? The game is balanced around a group of 4 and adding more would not do the game any good mechanic-wise. Adding an option with a visible alert that it most probably brake the game seems fine, but then again no designer wants to listen to people whinine "but the option was there and it broke my game!" every day after launch, no matter how many times you ask if the player is sure he wants it.