Not that we don't know what Master Chief looks like – he's described in detail in the 2001 novel Halo: The Fall of Reach.
"The funny thing about the Chief is he's actually really well described," O'Connor tells Eurogamer. "If you went down into that line and found a nerd with a deep canon Halo t-shirt and said describe the Master Chief to this police sketch artist, that police sketch artist would then produce a perfectly accurate rendering of an older man, almost painfully pale, almost albino white, with pale blue eyes, reddish hair, close cropped to a skin head, and maybe the last remnants of freckles he had when he was a kid."
O'Connor wants to keep the mystery alive and doesn't want fans to feel alienated by an official face. "That's a device to keep the player invested in the character and keep the player from constantly being reminded that they're not a hero or that they have to be a boy or they have to be a girl, or whatever that is," O'Connor says.
Maybe it's better this way, super fans. More often than not, on-screen adaptations of book descriptions are supremely disappointing.