It's not actually as cut and dried as that actually - it can absolutely be fooled without a face.You clearly don’t know how it works then. It needs a 3D representation of your face or some twin to unlock your phone. There’s a reason why there’s an IR blaster that allows faceID to be used even in the dark.
To make a 3D model of your face that is very similar to your actual face takes time and effort. Only those who are truly dedicated in stealing your phone would do this. And by the time you realize your phone has been stolen, I would hope some sense in locking/erasing your phone via Find My.
The thing is that it requires about as much resources as defeating Touch ID, so it's considered a reasonably good compromise between convenience and security.
You could argue in some close-quarters cases it is less secure as Touch ID since a dextrous thief can angle the device at the user then make a run for it while keeping the device awake...
...but again, to defeat it without a face present requires a good knowledge of iOS bypasses, and the change is that even if it were secured using some other less convenient method they'd have got around it anyway.