Who said it has to be on the back? Bro said that's what he prefers over Face ID, maybe just has never had one on the lock button.
Side button sensors work OK, too but I have much more misses on my supposedly more "modern" side button sensor phone than I ever had on old Pixels or any old iPhone with a home button sensor. I assume this is due to the size and general shape of a side button in comparison to an iPhone-style home button or old-Pixel-style back sensor which are bigger, indented and finger-guiding.