Why aren't smart people elected?

Instead of relying on charisma, why not have Presidents based on intelligence and knowledge?  Someone who has a masters or professional degrees, has written papers and books, and is just a really knowledgeable guy?  Rather than, um, someone who's not.

Jimmy Carter had a PhD, and an IQ over 170.  He was one of the biggest hypocrits we've ever had for a president, and accomplished little in his first two years, in his last two, he was viciously anti-communist; and I suspect very few of you would appreciate that.