I believe that a lot of this comes down to the American desire for legitimacy, combined with (usually unconscious) racism. American history is that of the native peoples, and is full of variety and interest, but doesn't have the deep continuity found in Old World history. This is possibly due to the more episodic nature of the North and Central American climate- the interplay of the Atlantic and Pacific weather patterns, the "funnel" created by the Rockies and Appalachians connecting the Arctic with the Caribbean, causing periodic violent swings in climate that have devastated civilisations before they took full root, and also partially explains their often psychotically violent world- view.

Desperate for richness to match Europe, and scornful of native cultures for failing to cope with the impossible, they interpret the ebb and flow of city- building cultures as evidence of the inadequacy of the natives, who are assumed to need Old World expeditions to teach them the techniques anew each time. So everyone from Sea Folk to Egyptians and Sumerians to Irish, Welsh, Vikings and Chinese are roped in as cultural missionaries, and every glacier scratch or weathering mark is taken as a Rosetta stone.

The Mormons adopted this attitude, and made up a book remarkable for its language (King James meets Jesse James) and for the fact that not even accidental resemblances to the events described therein have been found, despite the most intensive and massively funded efforts. They are an object lesson in the workings of religion.