[Guardian]

Guardian, actually that one's an easy one to answer. efficiency.
solar panels are great when they are stationary and the sun is directly on them.
but the greater the angle the less photons can be captured by the photovoltanic array.
driving a car around produces all sorts of inefficencies and makes it not worth while.

The majority of cars sit outside all day parked, whether you're at work or at home. Driving, for most, only takes up a fraction of their day. The difference would be the solar parking covers that a few places have now, or those that park in garages. Otherwise, there's still a good chunk of the day they're sitting outside. 

however the best way to use the sun would be to create a dyson swarm, beam that energy back to earth via a ring of receiver satellites in lagrange point orbits, then wirelessly transmit that power to vehicles via the road they're traveling on.

...and that's not inefficient?   The COST alone would be insane.