In these instances where their product managers make clarifying statements on the record, Apple likes to keep the answer short and simple and not go into any detail about the other, likely more real reasons. Something I miss about Jobs' presentations. He would often explain why things were or were not in a little more technical detail. And certainly with a lot more passion. These corporate, PR-type responses always make me cringe. They're rarely the whole story and are sometimes misleading.
That said, I don't think it's as simple as, "well your face is always in front of the display so it would be far easier than taking a moment to touch a specific button," as most people here argue. That argument is valid in certain scenarios for sure. I think the best example is instant authentication and unlock when opening the lid. Instant login to websites and apps might be another. What it doesn't address are payment and purchasing confirmations. With FaceID on the iPhone, there is a secondary action that has to occur (double pressing the home button) in order to prevent unintended purchases. In those situations, the Mac would need some sort of secondary action. I think Apple's viewpoint is that secondary authentication step is already there with the TouchID button, so why add another component into the mix (cost, software complexity, etc).
Not saying I necessarily agree with that, but my guess would be that's why they haven't. Or at least one of the reasons. There could be a technical reason, like not being able to fit the components in the lid. Or it could also just be a cost thing. I don't know.
The other thing it could be is Apple might not be ready or also unwilling to offer duel forms of authentication. Something I've been wanting and waiting for going back to the iPhone X. I'm hoping we'll eventually get there. There's always rumors that the iPhone one or two generations from now will add back TouchID. But, those rumors have thus far turned out to not be true. I really think multiple biometric authentication options makes sense from both a convenience perspective and a security perspective (something we know Apple is always happy to tout). But my guess is as of right now, it's either a cost thing, an implementation thing, or both.
But yeah, I'd bet money that some time down the road the reasoning they gave here will magically go away.
The secondary action would just be clicking the "Continue" or "Yes" or "I Accept" button.