Why has this history not been taught?

This is a rather long but awesome video about a pre-colonial culture which is not Aboriginal.  They are cryptically known as the Moundbuilders and basically dismissed.  But this culture was not isolated in the Americas.  The same structures exist also in Europe.  So what is really going on here?  And why, even though Abraham Lincoln and others of his era knew about these things, was this never really taught in school?  I think I would remember them talking about 9 -10 foot tall skeletons being found?  Check it out!  It lines up well with the fact that all of the ancient cultures knew sophistocated building techniques with monolithic stones and had an incredible understanding of both astronomy and mathematics. And this stuff was reported at the time in the newspapers!  I guess the press was different then.  Just sayin'