Because you either don't know how, or can't, blindshot with your phone.
basic steps on my camera:
0. phone is locked, but in hand. settings are set to be recalled when camera is launched. flash off, iso 400, -2 ev.
1. button shortcut to camera, a no-look move (enable it first)
2. aim and freeze your body, look sort of away
3. press the physical shutter button and wait for vibrate confirmation + time padding if it's an inaccurate indicator (volume button or whatever is the shutter button)
4. only now unfreeze your body and continue walking
5. lock the phone, but keep fidgeting with it
6. seem to use your phone, some phones open up widgets if you tap the clock on the unlock screen, so you can launch a nonphoto app eg weather by tapping it.
7. never chimp, never review images until home.
tip: use a wallet case, this way you can cover up the screen and still take photos.