Why does the US presidential gene pool seem so shallow?

There seems something distinctly "off" about the US political system when it keeps throwing up Presidential candidates that seem so underwhelming to the rest of the world.   I don't really remember when I last felt  particularly impressed by the qualities of a candidate.   I didn't even think Obama was impressive; he certainly seemed under-qualified, and his promise of "hope and change" was very much like the shallow sloganeering of the Kevin Rudd ascendency.  (Although, as it happens, I think Obama has turned out to be a pretty good President, after all.   His recent interview with David Attenborough showed an intelligent and decent man, even if his image is assisted by the comparison with the dimwittery that has enveloped his opposition.  His legacy in terms of health care reform, getting serious about some action on climate change, and on dealing with difficult economic circumstance, will stand him well in future, I think.)

Dismissing the Trump clown show, as far as I can tell Jeb Bush still seems the most likely Republican candidate.   As many have noted, it's funny how Americans rebelled against dynastic rule a couple of hundreds years ago only to more or less endorse another form of it now. 

Of course, everyone knows I follow the Krugman line that the Republicans have gone mad, and is currently a lost party that needs some very dramatic changes before it becomes  credible again.   But even on the Democrat side - I have never followed the Clinton family story closely, but remember how vigorously Hitchens condemned them, and I worry when any politician seems prone to self-aggrandising flights of exaggeration such as Hilary has displayed in the past.   (Shades of Reagan telling movie anecdotes, apparently believing they were true, if you ask me.   And no, I never thought highly of Reagan, even before it was known he was well on his way to dementia while still President.)

As for the only other Democrat candidate I have heard mentioned - Joe Biden??   Really?  He may be a decent enough fellow, but I had the impression he was mainly notable for making silly gaffes and had a distinct "Dan Quayle" feel about his vice presidency.

The amount of money that anyone needs to run for President in that country seems truly ridiculous, but I still don't really understand why that results in candidate runs by people who fail to impress.    Or is it just me, feeling underwhelmed ever since the last Kennedy was shot?   I do feel a bit hypocritical, because with John Howard, I sort of liked the way he was underwhelming in physical presence and in oration, but thought he displayed relatively sound judgement and decency and that this is what matters at the end of the day.  Perhaps it is because of the charisma of the Kennedy family that I feel the US leader should be impressive not just in deed but in appearance and campaign rhetoric too.