Why didn't Cao Cao accept Huang Zhong and Wei Yan?

Before Battle of Red Cliffs, Cao Cao was busy with Wu Dong. Liu Cong surrendered in name only in Jingnan four counties, and Cao Cao could not take over the enclave at the risk of being attacked by Wu Dong's navy. So even if he knew, he couldn't accept it, what's more, these two people, especially Wei Yan, didn't show much fame.