The "hidden messages" are mostly symbols of the Freemasons, which are a large part of American history. For instance, George Washington was a Freemason and was buried in his mason's robe. Washington, D.C. is literally filled with masonic symbolism. The number 13 is very important to the group...13 stripes on the flag, 13 stars above the eagle on the national shield, the eagle is holding 13 spears and 13 olive branches, etc. I imagine that most have simply forgotten what the symbols mean and so they remain on the money.