So... why don't we see fingerprint readers used in payments more often?
In short because they are not very secure. Fingerprint readers are certainly better than nothing when it comes to unlocking and locking your laptop or iPhone. When it comes to securely granting access to software, fingerprint readers fall short.
First, a fingerprint is simply not a secret. If a thief steals your iPhone, how many of your fingerprints are on the device? All it takes is a single fingerprint to allow a hacker to make a fake fingertip and defeat nearly any fingerprint reader.
Second, a fingerprint is an identifier. Like a username, email address, id number or badge, a fingerprint identifies you. For a secure computer application, an identity and a secret ensure that the holder of your identity has the right to use it. This is why passwords are used in conjunction with a username, email or other ID. Fingerprints are at best equivalent to a username, and they they are not secure without a password.