Anything with biometric authentication has to store the password in a recoverable way. The fingerprint sensor isn't repeatable enough to use your fingerprint data to encrypt the password, so all it can do is compare your fingerprint data to stored data, and if it's vaguely similar, give an okay to use a copy of the password it stored.

That's on top of a host of reasons why biometric authentication isn't very good security, so if you care at all about your data, just use a password. If you're any good at typing, it's usually just as fast and convenient, anyway.

It's slower on a phone, but a randomly-generated four-digit PIN's one-in-ten-thousand security is still better than a fingerprint, and most phones do allow more digits.