Because the main character wants to have power over his co-workers who always slight him or ignore him, but it isn't intended to be about sexual power over them. That of course could have been the story, but that would have been a much heavier, serious story to tell, and I imagine that the writers felt that a Black Mirror TV episode was not the place to tell that story.
Of course it can be argued that he has sexual power over them, but the writers likely didn't want to full-on go in that direction.
There are several reviews / articles on the episode bringing up Me Too, and labeling the main character as a misogynist, but he tends to hate, and to want to avenge both males and females. He started the game / simulation with torturing and playing out his power fantasy with his fellow boss, (and the son). So I'm not sure that his actions against the characters are specifically motivated because they are a woman, or more because each of the characters slighted him (though of course Nanette's slight was that she showed no romantic interest in him). And when Nanette was manipulating him, she was the person using sexual power, and he strongly and consistently resisted it. He did not take advantage of the situation and his power to act sexually or even romantically with her, even when given the opportunity by Nanette.
So it seems to me that the writers wanted to try to avoid the sexual power aspect or the character, as best they could.