A thief flags you down, grabs your phone and makes you unlock it using your thumb.
A cop opens the cop car door, grabs your hand and unlocks your phone, or even easier, face unlock.
Granted, guns and torture are rather effective as well, but is anyone entirely against fingerprint unlocking?
Pragmatically, is that really any different with a passcode? Someone might not be able to physically force an unlock like with biometrics by moving the relevant body part over, but there’s certainly nothing stopping someone from forcing you to unlock your phone if you had a passcode through by duress.
I rather doubt that, if in that kind of situation, there would be many who would resist. Your phone is not worth your life for most.