manufacturers can put it where your hand naturally rests, meaning that you can unlock the phone BEFORE you have even taken it out of your pocket.
Idk, my “unlock” finger naturally rests wherever the fingerprint scanner is on my phone. When I had a rear fingerprint scanner, I used to have my phone’s bottom right corner planted into my palm near the thumb and used the index finger to support its back near the scanner, so I was always ready to unlock it.
Now that I have an under-screen scanner, I use my pinky as a “shelf” for the phone’s bottom side, ring finger to hold it on the far side and index finger along the near side (which makes me suspect this grip would work for in-power-button scanners too), and that makes my thumb naturally rest exactly on the spot where the scanner is. With (one) tap to wake, I have no problem unlocking the phone while taking it out of my pocket - literally just a quick double tap. Although it’s true that you can’t unlock the phone directly in the pocket like this, because the proximity sensor should prevent the tap to wake from working.
I used to have a phone with a scanner in the power button too, but I can’t remember how I held it - I don’t think it was the same way as now, because I’m pretty sure I never used to rest my thumb on screen like this.