I don't believe there is a technical reason why they can't do this. They MAY have some security concerns, but I think certainly they could make it work. I know on my Samsung phone I have the ability to setup safe zones, that is my phone knows by location (home, office, shore house, parents, etc) that I am in one of my "safe zones" so it's ok to not use the lock. It can also detect my voice, when it's on my body, AND when it's connected to a trusted device (apple could leverage THIS to unlock a Mac), and again not use the lock. So the tech is out there.
I DO have an Apple Watch, and I love how it unlocks my Mac Book! I agree this would be handy if the phone could do it also. Best suggestion I could give would be to send Apple Feedback on it.....
https://www.apple.com/feedback/#mn_p