How come the U.S. has produced so many false prophets?

Prove that Joseph Smith was a false prophet and your question might even become intelligent.  Try not to beg the question.  He correctly predicted the American Civil War 30 years before the fact.  He was ridiculed for it because "everybody knew" the Union was too strong for that.

Harold Camping made *millions* of dollars.  Joseph Smith convinced more than 33 women to marry him.  There are incentives to lying to gullible people. 

Because all prophets are false

It didn't until a few hundred years ago, when one proud misguided Catholic priest thought he could do a better job of founding a church than Jesus Christ did. So he left God's Church and founded one of his own, supposedly based on "the Bible alone", which of course meant his personal interpretations of the Bible. Before Father Luther died, there were already half a dozen other Protestant churches that had broken away from his church because they didn't agree with his biblical interpretations. The doctrinal chaos of Protestantism had begun, and continues to the present day, over 6,000 denominations now existing, eat one claiming to follow "the Bible alone", yet the teaching of each one contradicting the teaching of the others. Meanwhile, the one Church Jesus founded (which includes well over half of all Christians) remains one in belief, one in teaching, one in worship, one in biblical understanding throughout the world after 2,000 years. You just can't beat God's plan.

False prophets are not limited to the USA. Buddha, Mohammad, and all the Eastern religions all can be founded on false teachings and/or false prophets. 

"False" is the only type of prophet there is.

Since I'm a Christian and I've never heard of those "prophets", I say your claim is bogus.

i dont think thats true

You mention Russell.  JWs do not claim he is a prophet.  You need to learn what that word means.  An example of a prophet would be Isaiah.

Furthermore, why don't you include Billy and Franklin Graham, and other religious leaders in that group?  

Because there's no other kind of prophet.