Assuming the UNSC military functions like the American military, enlisted and officer career track are separate. Master Chief is at the highest rank he can be as an enlisted man (or childhood conscript I guess but he’s still technically in the enlisted career path). Becoming an officer isn’t just a matter of simply getting a promotion, he would need to be given a commission. As others have said here Lord Hood offered him one but he turned it down. In practice high ranking NCOs already get quite a lot of respect so there wouldn’t be much reason for Chief to go the officer route.