If someone is willing to cut off your finger to access your device, that means they have access to you and the device and the means to convince you to unlock it.

Cutting off someones finger to access their phone is like using high explosives to blow up a door you can easily kick open. Criminals actually don't want to get caught.

Maybe the first one won't, but the next time they most definitely will. Stealing a phone is one thing, cutting someone's finger is another risk-wise. So if you have to dramatically increase the potential penalty for getting caught, you'll make sure it actually works before doing it repeatedly, and this kind of info is shared among thugs