Because it is not a very healthy habit to let reptiles of any kind to get near your facial area. It's sort of a no brainier. It's not sanitary or "good hygiene". And after years of keeping large constrictors it could be very hazardous. Sorry to say this, but you think an experienced keeper would be aware of these things. Not to mention if it did decide to bite you, your face would make an excellent target. Remember And anything that slithers in its own waste is not something I would want by my head in general. And with your mouth emitting all that heat, what a target. Need I say more.