That’s because there never was a “Super Bowl I”. The name was applied retroactively.
Before the 1970 merger between the American Footbal League (AFL) and the National Football League (NFL), the two leagues met in Championship contests. The first two were marketed as the "AFL-NFL World Championship Game". What is now known as Super Bowl III in January 1969 was the first such game that carried the "Super Bowl" moniker in official marketing; the names “Super Bowl I” and "Super Bowl II” were retroactively applied to the first two games.
So, in reality, the first true “Super Bowl” game was played between the New York Jets and the Balitimore Colts in 1969.