Signature is also not something "you are" - it is an easily faked thing, that is basically never actually verified. PINs can be changed, so if you came up with

Signature is also not something "you are" - it is an easily faked thing, that is basically never actually verified. PINs can be changed, so if you came up with a way of memorising them, its easy. You also don't need PINs for things like loyalty / membership cards traditionally - just for payment cards.