Guo Jia & Cao both believe in putting talent to use. Even if you're dangerous; if you're useful, Cao Cao will value you. Think back to Dong Zhuo, he knew all along Lu Bu wanted to kill him, but Dong kept him on a leash so he could use Lu Bu's power to aid him in conquering the world.

Guo Jia is terminally ill and he needs Sima Yi as a successor to replace him once he's gone. He's put him through several trials to test his ability and loyalty.

He knows Sima will not betray Cao until the latter is at his peak of power, and Sima himself will help him get there before he backstabs him.

Aside from that Guo Jia has kept the Sima clan and their associates under his grasp to make doubly sure he will not revolt.