My guess is that you need BT to put the lock into “program” mode so that you can add or manage fingerprints to it. Otherwise they would need to add a UI or at least buttons to the lock itself. Unlocking with your phone is probably just a side benefit. Most smart locks also have time controls so you can allow access to specific codes / fingerprints only for certain times of day. It is way easier to implement that using a phone app with bluetooth than trying to cram enough buttons into the padlock to do that directly.
