I teach this as a way to get into good habits. If you keep putting your mask on your head chances are a wave or someone’s fin is going to hit and and you might not get it back. You might throw your head underwater to adjust your BC, wetsuit or fins...mask gone. This is from my teaching experience and the student said..."you told me so". If you are bobbing up and down with your mask on our head in 6 ft seas in the Mid-Atlantic the DM might swim out to you and be pretty pissed for the false alarm. This was another experience. And lastly mask-on-forehead contributes to fogging in some conditions from the heat loss from your head in the ambient air.
Overall start good habits early. When you become a super diver you can do what you want but students and newer divers need some guidelines so they don’t look stupid after losing their mask on dive one of a 4 dive weekend. Yes diving...it’s all about how good you look.