My opinion: Because it is easier to reguritate handed down info than to put individual thought into something.If this is the case, then why do most of the certifying agencies teach it and so many divers go by it as a rule?A mask on the forehead is NOT a sign of distress. That's one of the biggest myths in diving.
I tell my students to leave their masks on their faces for the duration of the pool session (to get used to it) and in open water (so as not to lose it) based on my own personal past teaching experiences. I have also had to retrieve lost masks for non-students for the same reasons, but I am not their instructor so if they lose a mask because of poor handling with it, well that is their choice. I just tell them if I find their mask first, I get to keep it...
And I agree with diving with a back-up mask and I always have a second one in my float when I am teaching to avoid having to take a whole class in if a mask is dropped over deeper water.