We can fly ultralights without a license. Granted those "vehicles" technically aren't airplanes and generally only result in loss of life to one's self.

"We don't have even the political willpower to take away licenses from obviously unfit seniors"

I believe many states do require a retest after a certain age.

"or making the tests hard enough that way fewer people can get one in the first place"

Fewer, sure. Way fewer, maybe not. I think most people are capable of safely operating a car. I think there would be a large percentage that would just need to put more effort into learning. The way it is now is that people remain ignorant on the subject because they aren't required to study and learn, because licenses are practically given away. Plus, why waste time learning the law if nobody else follows it either?

Tougher licensing isn't (or shouldn't be) primarily about depriving people of licenses, but rather providing forced incentives to learn through testing standards. The standards shouldn't be so high as to prevent most people from driving. If they are then we could change some of the rules or designs to simplify the process, although likely at the expense of rate of flow.