Freemasonry is based upon the stone masons guilds of medieval period England
Many of the Founding Fathers of the United States were well known to be members of Freemasonry. This was fairly common for English men of the upper classes at this time.
The imagery of the "All-seeing Eye" and the pyramid, unfinished or otherwise, were common and popular imagery in the 18th century. They are not unique to Freemasonry at that time.
The motif of the number 13 is repeated on The Great Seal of the United States because there were 13 original colonies when the nation was founded. The fact that 13 is an important number in some parts of Freemasonry is coincidental.