The reason for that is that if everyone knows that it is Monica then what is Monica trying so hard to protect. The struggle for Monica is to hide her current situation from the world and with every future scene more and more people are finding out the truth. The struggle of preventing something from happening but it happening anyways is what is humiliating in Monica's situation. It is exciting to watch as little by little her reputation is getting destroyed. This update Stephanie and Rebecca saw Monica do things for Victoria.
Steve knows that it's Monica. Whether his secretary knows that or not doesn't make any difference. Julia knows the real identity, Philip does, Fred too. With White the dislike is mutual. She hates the person Monica and would never have her work for her photoshoots, we are given a glimpse of that in the previous audition. She doesn't trust Monica. So in order to have Monica be humiliaited by white such a Dynamic was necessary.
Plus the real enjoyment is the player knowing that it is Monica getting humiliated by the people she looked down upon and the people she hated. The employee you mention, we don't even hear such thoughts of his, that he is ecstatic about fucking his boss. He is totally scared and will always be subservient to Monica. In his eyes Monica is woman who can wipe him off the face of the planet in a minute if he dares open his mouth. In other scenarios since the characters don't know it is Monica they interact with her more freely and treat and talk to her like a whore which is pretty degrading.
So there is a plus to the path chosen by DM. Maybe it doesn't work you. Also i do understand that continuosly repeating this trope in every storyline is very redundant. DM should find more imaginative ways of writing his storys and progressing the plot. It is undoubtedly irritating when every character acts dumb and immediately considers the possibility of Monica's look-alike.