It has been brought up before and the problem with it, is it fundamentally changes how people act. Which would mean every single scene with one of those NPC's would need two versions or more, under the therapist control or not under their control. Then really you would need more since it is a gradual thing that eventually ends in total control.
Also while I won't spoil things it doesn't really fit the story, the therapist is not looking to make a harem he is searching for the right girl and once he finds her, she is the only one he is interested in and the right girl is Sveta aka you.
So in theory in the early stages before he decides Sveta is the one, it would make sense he would see others for awhile and do the same to them. But honestly it would be a ton of work for not a lot of payoff, since it would only be for awhile and only a few people would see it. So this is just one of those we feel the amount of work for the payoff just isn't worth it, when we could put our time into other area's of the game.
I mean the game is suppose to go 8 in game years and we are still working on year two with still stuff in year 1 that isn't finished.