It would be nice, but the phone would have to be unlocked at the same time you try to authenticate on the computer for this to work. Part of why it works with the watch is that you have to have an unlocked (but pin-lockable) watch near the computer. Most of the time when you're wearing the watch it will be unlocked (this is the default behavior, after unlocking it stays unlocked while worn) whereas the phone will relock itself. So it would be useful, if you happened to have already unlocked your phone near your laptop, but probably not as consistently useful as the watch. Most of the time, you'd find yourself unlocking your phone to unlock your computer which is not really convenient, so you'll just unlock the computer itself.

This would be useful in the case when laptop lid is closed. TouchID/FaceID are still very fast

The Magic Keyboard for Apple Silicon macs has a Touch ID button. Works great.

You can do this with a chromebook and an android phone. I find it is more comfortable and faster to just type in the pin vs unlocking the phone, so I don't use it much.

MacOS doesn't believe in PINs. It's passwords or touch id.

I'm glad my windows PC only requires a PIN :)

It’s not that different than initiating an Apple Pay for. Safari on Mac and reaching into your wallet on your iPhone. The framework is there. The UX would be to get a prompt in a similar way and Face ID unlock