Because if he took off his helmet, there would just be.....
..... another helmet underneath!
It's funny you'd say that, because in Halo: CE PC, in the end of the campaign Chief takes off his helmet and the camera pans up so you can't see his head. Modders found a way to rotate the camera, and it revealed that after taking off the helmet, there was another underneath.
"Knowledge speaks, but wisdom listens." GT: RAGEface14 (I DO NOT accept random requests)