Probably because phones hadn't been invented yet and the founders couldn't get hold of Miss Cleo? </sarcasm>
Actually, the states were originally enumerated. The Committee of Revision (aka Committee of Style) - led by William S. Johnson (CN), joined by Alexander Hamilton (NY), James Madison (VA), Gouverneur Morris (PA) and Rufus King (MA) - made numerous changes. Morris is the one who changed the Preamble, since it was unknown which states would ratify. Additionally, denoting the states would require Constitutional amendments to revise the Preamble with each new state added.
Why not describe it as an agreement between the states as the Articles of Confederation did? Because it was ratified by the American people and not the people of Virginia, etc.
Wrong. It IS described as an agreement between the states (Article VII - 'The Ratification of the Conventions of nine States, shall be sufficient for the Establishment of this Constitution between the States so ratifying the Same.') And one would have to be insane to aver that it was ratified by the people en masse - can you point to this super-ratification made by all states in unison???
A lame reply, but I can see you're struggling. Then why didn't the Preamble start "We the People of the Several States"? That would negate your amendment requirement. Instead it starts "We the People of the United States" making it clear that it was in that role the peope would ratify the Constitution, and that the states were not.
It IS described as an agreement between the states
No. If so then the Constitution should read "Ratification of nine States, shall be sufficient for the Establishment of this Constitution..." Instead, as Chief Justice Marshall pointed out, the people of the United States, meeting in conventions held in their respective states, ratified the document.