Because it works for authentication too. My password manager just... automatically authenticates me without me having to tap a thing. It recognizes the login form on the validated domain, it scans my face, it fills in my info. Same as paying with Wallet, I just slide up the credit card I want to use and it scans my face as I hold it against the reader.
And I'm not always pulling the phone out of my pocket, I'm picking it up off the table. I grab it by the edges, I'm not putting my finger on its back.
The fingerprint sensor requires me to put my finger on it.
FaceID just works without me doing anything. I don't even realize I have to authenticate, and then have to do something in response -- it just does it.
If you haven't had a phone like this before it's hard to explain. But I can tell you from personal experience that it is less effort than FaceID, which is my current daily driver