First of all, Cao Cao is suspicious by nature. It is Cao Cao's famous saying that I would rather be called a villain than a villain. If Cao Cao knew that Sima Yi was unfaithful, he should have decided to destroy Sima Jia long ago. On the other hand, Sima Yi is a sly old fox. He naturally knew the character of Cao Cao's father and son, so on the one hand, he was careful not to touch state affairs as much as possible, and then he tried his best to assist Xelloss and gain the trust of Xelloss, so as not to let Cao Cao kill him.
The second possibility is that Cao Cao and his son knew Sima Yi's thoughts, but at that time, Cao Wei had just unified the North when employing people, and needed a group of prestigious literati to stabilize people's hearts. It happened that Sima's family was an aristocratic family at that time, which could win the hearts of the people and stabilize them. Besides, Sima Yi is really capable, and Cao Cao is a master who cherishes talents, so even though Cao Cao may be aware of Sima Yi's infidelity, he wants to keep him first.
After all, on the one hand, people's hearts are stable, on the other hand, there is still a shortage of talents, which is not worth the candle. Besides, is it possible that Sima Yi really turned against heaven? So after weighing the pros and cons, I didn't kill Sima Yi! But I think that no matter what Cao Cao and his son thought at the beginning, history is a foregone conclusion. No matter Cao Jia or Sima Jia, they all left a strong impression in history, that's enough!