My guess is the cost is too high. First Adwords is an auction. Then you have to deploy to the billboard which requires an ad of suitable format and then someone to design and put it up. Maybe electronic billboards would work? I would think adwords inside video games would be good.. or live tv events (e.g. soccer matches).
I thought Vistar Media (some of the Invite Media co-founders) was doing that several years ago. It appears they pivoted, which might be a bad signal.