What stops you from slapping a solar panel on the roof if it is the most efficient way to charge an EV?

the fact that many new electric cars do have solar panels, I fail to see your reasoning. A solar panel isn't going to provide you with enough energy to drive. Merely enough energy to trickle charge your batteries.

Now, should you run out of charge during your drive, you simply have to wait a while and you'll have enough to get you to a charging station. Or you can walk, taxi there. On an ocean or channel crossing, you don't have that luxury and must rely on other ships if you run out of charge. The point I'm making is that any electric vehicle should incorporate solar panels into the design to minimize it's dependence entirely on the batteries and can extend it's time doing what it's designed to do.

As solar panels advance and the wattage increases, this will be more and more important as it will open up new avenues for transportation. Like the solar LSA plane "Solar Impulse" that can fly indefinitely.