Preamble to the United States Constitution
At the time, Gouverneur Morris chose to write "We the People" instead of saying "We the United States of..." and then listing all thirteen states.  First of all, Article Seven of the Constitution said that only nine states had to approve the Constitution for it to take effect.  At the time, nobody knew which states would ratify the Constitution and which would not.  Because of this, the Preamble could not list all thirteen states as if they had all already agreed.  Also, if other states joined the United States later, it would be impossible to add their names to the list.  For these reasons, the Preamble had to start with "We the People."