Koufax retired at 30 years old. I can only go by what he did in those years. He had nearly as many strikeouts as Spahn did in almost 3,000 less innings pitched. If you ask any player in the history of the game who they would rather face Spahn in his prime or Koufax in his prime i'd be surprised if 1 player said Koufax.
I don't think anyone in their right mind would try to argue Spahn's peak over that of Koufax. But my point is, much of Koufax's value came from that 6 year stretch whereas Spahn was a very good pitcher for nearly two decades.