Except that a regular pin pad lets anyone enter the pin. Your pin code can only be keyed in by 1:1000000 people [citation needed]. So no, your pin is not on your forehead. Your pin is an organic material with color and depth and movement that for all intents and purposes is your actual forehead.

The average opportunist thief won't be able to duplicate that key. The best that they can do is use your actual face, within a few feet from you, while you're staring directly at the phone in their hands.

Funny you should say that, here's a video of a guy accidentally unlocking a phone and using his apple pay by pointing it at him https://youtu.be/WYYvHb03Eog?t=1m27s