At first I was disappointed by the lack of Face ID, but now I understand why it wasn't needed:
1) To unlock an iPhone you pick it up, wake up the screen, and then it scans your face. This physical action is required before a face scan happens. There is no equivalent on the Mac. You have to tap the keyboard or click the mouse to wake up your Mac. If you're already tapping a key, then it might as well be the fingerprint reader.
2) When making online purchases on an iPhone, it scans your face, but still requires a click or two on the side button. There is always a button press involved for verification. Some kind of verification is needed on the Mac too, and if your hands will be clicking some key for verification it might as well be the fingerprint reader.
3) I definitely don't want my Mac continuously scanning for faces either. Then it would wake up every time I was in front of it.
I think the only case where Face ID would be handy on the Mac is when you physically open a laptop that is sleeping. However, if this is the only useful case, then heck, I'll gladly just press the fingerprint reader since my hands are already near the keys.