Yup ego is one. Production reality is another, especially if you talk about big budget SFX extravaganza. Indie films are better and you usually do see only one writing credit, a lot of times the same person who directs it. They have more control of production budget, etc. I mean, if they know they only have a $5 million budget, they would do the script a whole different way. Also, the production goes through fewer hands.
For a big budget movie, it is a totally different beast. You have everyone from the executive producers and producers (who pay for the production, by the way, so they have POWER) to the director to the actors who all want a say in what is being produced. Like someone said, the script was only a blue print. By the time it goes into production, it MAY not even resemble the original. And then you have last minutes changes based on whatever factors such as production conflicts, difficulties, budget ballooning, etc. and suddenly the producers say, "You can't do that. It's too expensive or I'll shut you down" or Hugh Jackman telling you, "I'm not doing this. It doesn't make sense to me." Or 10 million other reasons why the script would be changed at the last minute (one story is that Stanley Kubrick changed the script of The Shining so much during production that Jack Nicholson didn't even bother to memorize his lines, cuz he knew they would be changed the last minute anyway.
The major problem is, a lot of these top guys (producers, etc.) are NOT creative people. They look at everything from the money angle -- what they think will make money: Oooh, it's the Transformers, so give them more explosions, epic fights and sexy babes. Forget about plot. Give them an extended 30 minutes fight scene! A lot of them do not know art or storytelling if it smacks them in the face. Dialogue? What dialogue, just have them info dump. Character development? We don't have time for that. Etc. etc. So again, by the time they're done with the shooting script, the great stuff has been stripped off in lieu of all the big, overblown action-adventures.
And then there's the other end of the problem: the script starts off as total fluff to begin with. Again, those involved with the process are not necessarily creative types, so they only go by the money. Pitch them an idea that sounds like a lot of cash, and you sell the option. Then comes the pressure to deliver what you promise so you go with the sex and violence and big budget explosions and car chases in place of character development and plot because you run out of time. They only gave you three months to finish the script.