The huge advantage of biometrics (fingerprints, FaceID, etc.) is the ease with which a user can unlock their phone. A passcode may be better than a fingerprint, but a fingerprint+longer passcode is better than a shorter passcode (or no passcode at all).
Having a 12 character alphanumeric passphrase you enter each time you want to unlock is not something most users want to do.
See e.g.: https://www.businesstoday.in/technology/news/story/what-kick...
Only about 49 per cent of the users were setting a passcode, which meant that the remaining 51 per cent were not benefiting from the data protection mechanism. When Apple dug in to understand the reason, the findings revealed that users unlock their devices a lot - on an average about 80 times a day. And about half of its users simply didn't want the inconvenience of having to enter their passcode into their device, at times. At that time, in 2012-2013, the default passcode length for iPhone was four digits, which happens to be six today.
Apple realised that it needed to come up with a mechanism that's fast and secure, and doesn't involve typing in the passcode. That's when Apple introduced Touch ID, which was easy, fast and secure. The way that biometric authentication worked on Apple platforms was that the user must set a passcode to be able to use the biometrics. And just as Apple thought, there was a much higher adoption of biometric-based TouchID. Apple says over 92 per cent chose to use Touch ID and had therefore set the passcode, which in turn meant users were able to use Apple's data protection encryption system.