Anyone can act, but not necessarily well, and not necessarily in a manner that people like. Those who can act well will be in higher demand. Those in higher demand can demand higher wages. Those in Hollywood can offer the highest wages. It's simply capitalism at work
Collaboration is not necessarily a bad thing. As a director, I would be pretty pissed if an exchange went: "Listen, I really liked the script, but there are a couple things that I wanted to change..." "Hey **** you." As a producer, they would probably end up being fired after ignoring the words "completely over-budget" for about the fifth time. Of course by this point, as an actor, I could very well be completely clueless as to how I was supposed to make a sweeping gesture with my right arm when it had clearly been lost from bomb shrapnel five scenes ago.
Now granted, not every movie may have such colorful examples, but there are reasons for the script getting changed. In many cases, I imagine that the writer said "Gee, it really worked well in my head, but when I saw it onscreen, it totally works better that way" Not that they would ever admit it, of course.

Having the balls to do like everyone else in your own group is doing is not really having the balls at all. I'm sure I can come up with some new idea that hasn't been done, such as a story about Custer Flapjacks in the grand star cluster of Darwinia, which is ruled by the naive yet peaceful Dinosaur race that emerged from the womb of Mother Sheriff, the galactic apple tree, three zeellusssions ago. The end of the movie would be the powerful yet graceful herd mother dinosaur (literally) torn about by spontaneous insanity, raping The Colonel to death as he fought back with a bendy straw.
I doubt many people would appreciate that movie, though...