First of all, Cao Cao and Sima Yi's father Sima Fang have a good relationship. Cao Cao used to be a subordinate of Sima Fang. They had a good relationship and often met and drank. Because of this relationship, even because of the relationship between Cao Cao and his father, Cao Cao could not have killed Sima Yi. After all, the former friendship was there, what's more, Sima Yi was really a talented person at that time, and Cao Cao was a talented person, just like the reason why she didn't kill Liu Bei at that time, all for recruiting talents in the future.
Second, the relationship between Cao Pi and Sima Yi is very close. Cao Pi has always trusted Sima Yi very much. Even if Cao Cao wants to attack Sima Yi, there will be obstruction from Cao Pi. Moreover, Sima Yi was only a small official with a little talent at that time, and was not a threat in Cao Cao's eyes. What's more, xelloss is his son and can inherit his position in the future. For a talent like Sima Yi, there must be only advantages and no disadvantages in helping around. The most important thing is that Cao Pi is also very powerful and can contain Sima Yi.
Third, and most importantly, Sima Yi experienced several emperors, but because those emperors were short-lived, Sima Yi was always cautious and promoted all the way, and finally became a general in title of generals in ancient times. At the same time, he also holds various important positions, so to speak, he is in power. It was not until he was sixty years old that his ambition was gradually exposed. This is because Sima Yi was a little talented, but several emperors before him were also very capable, and they could kill him when they were young, but they didn't realize it.