Well, the main reason I see. Is that would just put her way too much in the spotlight, and that's not really her style. She clearly likes to remain in the background, pulling strings and manipulating people safely from the shadows. It would hinder her a bit IMO. If she was openly attempting to seduce the MC and trying to become pregnant.
Also the fact that she's been his teacher for his whole life, and the reason for him existing to begin with. So it would be a bit weird to people if she was suddenly now trying to seduce him.