'Cause we've always done it like that, since the state's second constitution was adopted in 1784. (The position was called President until 1790.) Same reason there is an Executive Council: to limit the power of the Executive. Partially in response to having the longest serving Royal Governor for 25 years: Benning Wentworth. It would take a constitutional amendment to change it, making that unlikely. Vermont is the only other state with a two year governor term. And they only switched from one-year terms to two-years in 1870.