Because that's the only way a 19 years old can win against the peak tier of One Piece world in the shortest time.
Had Oda not hit his head and continue properly, as in, 2 big arcs for "Luffy as Emperor" Saga, then Luffy's progression can increase naturally.
Nika DF had to be a myth zoan (bonus all around) and give increased strength, to compensate for Oda rushing his story.