Nothing is impossible in football, but why would Klopp go to Utd? His wage is probably on par with Moyes' if not higher considering he's got a good billion of ad contracts here in Germany (from electric shavers & glue to cars and insurance). His image / personality is that of a loyal, down-to-earth guy next door you'd like to hang out and drink a beer with. He'd lose all that. Leaving out of the blue after having just extended his contract would completely ruin his reputation in the German public eye.
Also his near-mental-breakdowns would not sit well with the British public & FA and the shit press would tear him apart & Hitler-ify him.
Here he can completely shape the club the way he wants, he can make Dortmund his legacy just like Fergie made Utd. his. Why would he want to always be compared to Fergie and deal with fat, old balding stars?
Currently he's basically guaranteed to come in 1st-3rd in Germany - thus to compete in CL and has a legitimate shot at winning titles. He'd have nothing for granted at Utd. Even the big pile of money he might be given pales in comparison to the oil clubs and there are even more legit contenders in Liverpool & Arsenal and then some.
Apart from that he could walk in and out of jobs at almost any given club, why choose Utd. at this given time? When he decides to leave in 2016 or 2018 Utd. will probably still be in disarray & chaos. A second Fergie is hard to come by.