Of course, Sima Yi did this because he felt that teaching Cao Zhen was troublesome; it was better to give him a core, and let him figure it out himself.
This way, if there were any problems during the process, he could naturally deflect by saying, this is your understanding of the military formation, and it's your deductions. Mixing other inputs would spoil it, thus effectively dismissing the issue and allowing himself plenty of time for research.
After all, Sima Yi was also very busy, with many things to attend to, and in his opinion, Cao Zhen was a bit obtuse. Little naive child, play on your own; I'll strengthen myself as well. I have many things to handle too, and Zhuge Liang, this bastard, always gives me the impression that he secretly plans something. I need to make some preparations.
