Because usually script writers grab a perfectly good story and to adapt it to the movie turn it into some horrible story that has little to do with the book and if littler to do with the world.

Just take into consideration that they wanted to make a sequel to the film Gladiator (Great movie, good story-telling, etc) and the writer wrote a story on how Maximus goes into the Roman underworld then tricks the gods into bringing him back alive and gains immortality, proceeding to go through history doing justice (Protecting Christians from the Roman persecutions, fights with Crusaders against the muslims, fighting in the World War 2, fighting Vietcongs in Vietnam, working at the Pentagon, and commanding Mecha-bots into battle.)

...So yeah.