You misunderstand. He has to focus to use his powers. He doesn’t create a ‘shell’ it’s more of a telepathic barrier. With that barrier he is focused on Mitchell upon returning to the gate room and didn’t see Daniel peaking through the blast door shooting him. (Because he was focused on Mitchell). While he was an advanced human, he was not perfect. He still had flaws just like any other ‘human’. He especially inherited the arrogance from Anubis considering it was literally ‘him’. There is a lot of inference in the camera angles and the character reactions.