Why are a bunch of history professors considered such experts in rating presidents?

Every year, a bunch of historians and history professors rate presidents, who the greatest was, who the worst was. Lincoln is usually number 1, FDR is usually 2, Warren Harding is usually the worst. My question is, why are these historians and history professors considered such experts? In reality, isn't that just a matter of opinion? In my opinion, Ronald Reagan was the greatest president in history and Jimmy Carter was the worst. How come normal people never get to rank presidents?