Because the point of a rookie QB is not to win as many games as possible. You want them to learn how to play, and throwing quick checkdowns 18 times a game is not how you learn to play.
I remember a huge point of contention was that Josh was throwing too much in 18 and 19. But I firmly believe that's in part why he developed so quickly.
Running the ball 35-40 times a game and only calling 20 passes is how you convince yourself EJ Manuel may be the next franchise guy after 2 seasons.