Why doesn't Tesla have an eye tracking camera that ensures the driver is watching the road? It would cost hundreds of dollars at most. Basic eye tracking is a solved problem and would ensure drivers are actually paying attention.
If you still want to use your phone, it'll be easier for the cops to catch you doing it.
[0]: http://www.cadillac.com/world-of-cadillac/innovation/super-c...
[1] https://eu.detroitnews.com/story/opinion/columnists/henry-pa...
Simply saying that it is ineffective is not an answer at all. Anyone with access to YouTube can see eye trackers working remarkably well.
Maybe he simply wasn't willing to disable Autopilot for users that could not be eye tracked accurately (glasses, etc), but that seems like a very bad reason. They should start with the ideal cases and add support for wearing glasses or low light over time.