Americans do two kinds of comedy. Complete crap or utter excellence. Seinfeld fits into the latter category. Good American comedy doesn't rely on Fawlty style slapstick, double entendres or cheap caricatures. It's much more subtle and nuanced. Considering the general image of Americans as loud and trashy, their penchant for great comedy is nothing short of amazing.
By the way, if you think Seinfeld is a fluke, take a look at the Larry Sanders Show. Nothing produced anywhere has since come even close to this level of excellence.