Some people want the option, like how androids have face unlock, fingerprint sensors, etc and it’s been talked about many times now that I guess apple is giving up on the under display Touch ID. Some people would rather pick up their phone and have the fingerprint scanner over Face ID. It’s just nice to have choices on biometrics. For me personally I like my Face ID, but not gonna lie sometimes I wish I could just tap a finger and be in as well if I don’t feel like lifting the phone up, or to the proper angle to scan my face. Plus they have the hardware already to put it in a small pill shaped sensor like the power button on the iPad Air so it’s theoretically possible that it can just be a cut out kinda like how the millimeter wave antenna oval on the side looks.