Having a single chief magistrate is good for practical reasons, too. The absolute power of the executive is limited by the constitutional power of the Congress to create the executive departments, and the requirement that the President seek the advice and consent of the Senate when appointing officers of those departments.
In fact, the Virginia delegation was the only delegation to the constitutional convention which arrived with a plan. According to their plan, there would have been a plural executive, chosen by the legislative branch. The delegates very quickly rejected this portion of the plan. In the Continental Congress, the President of the Congress was the closest thing to a chief magistrate which the nation had. He was hamstrung by the power of congressional committees, who could keep him on as short a leash as they pleased. As he was chosen by the members of the Congress, he was beholden to them, and no end of intrigue accompanied the backroom negotiation for the position.
While the convention sat, the delegates looked to the front of the room each day and saw Washington sitting there, the President of the convention, by the choice of the delegates. No one there had any doubt that he would be the nation's first chief magistrate. Sadly, few of his successors have been able to meet his standards of personal probity and natural leadership. But the office was crafted with him in mind, and physically before the delegates each day. This is not to say that he created the office--he failed to get much that he wanted, such as a three-fourths vote to override an executive veto, and the power to create his own cabinet officers.
A lot of the questions you ask about the history of American governance, Ogionik, could be answered if you carefully read the declaration of independence and the constitution. I would recommend this page for an annotated hypertext version of the constitution.
I would recommend the "Thomas" section of the Library of Congress web site
for historical documents and in-depth information on the promulgation of the documents and their significance in American history. The Thomas section also contains archives of the history of Congressional legislation, so that if you were to become familiar with it, you could find out how the Congress works, how laws are made, and how we came to have the government we now have.