Because the "presidents" before George Washington didn't hold the office of President of the United States, as created by the U.S. Constitution to be the head of the executive branch.

The previous "presidents" were members of the legislative branch. The office was the "President of the Continental Congress" and was an entirely different office that just partially shared the same name.

The President of the Continental Congress is more the spiritual predecessor of the office of the Speaker of the House, although that's not entirely correct either since the Continental Congress was a unicameral legislative body, as opposed to the bicameral Senate and House created under the Constitution.

In short, the Thirteen Colonies operated under a different government until ratification enacted the Constitution in 1788. Any office before that that had "President" in the name was a separate office unrelated to the office of the "President of the United States".

And for the record, Peyton Randolph was the first President of the Continental Congress, in 1774. John Hanson was the ninth, but happened to be holding the office when the temporary Articles of Confederation were enacted in 1781.