The hosts were the control group, and if anything about the control group changed, the whole experiment would need to be changed.
The goal was always to track and monitor the choices each human being would make when confronted with almost identical hosts and almost identical situations.
The main goal was never to test the hosts and see how their personalities, functions and movements would hold up when interacting with real human beings.
Nor was the goal to constantly refresh the storylines in order to make it as interesting as possible for returning guests.