Because the land needed for production to meet demand would be huge, which means this land can’t be used for food growing and adds to pressure for things like forest clearance for new fuel crop growing areas. It can only ever contribute a small amount to the picture, it’d never be possible to run every car on it (although if everyone uses electric hybrids running on electric most of the time then the demand might fall and it could contribute a larger amount to the remaining fuel used for long journeys only).
I do wonder if there will ever be companies that could retrofit electric power plants to existing non-electric cars, maybe for common models like the Focus. It’d save a lot of the energy wasted in new car construction, but would need the batteries and kit to be small enough to fit in the existing shell, and maybe need new controls and so on. It’d be interesting to run something like this as a competition to see who could do the best job, one for universities maybe.