A screenwriter friend of mine once said that describing a screenplay as a "blueprint" for a movie was horribly inaccurate. He likened it more to the travel guide abandoned in the hotel, that thing that you purchased with a great deal of enthusiasm and which may very well have actually gotten you on the plane and to the beach, but by the time you were there you had more important things to do than stick to the plan so while the book may be full of highlighted passages and dog ears, it really has only a tangential relationship with your vacation.
Granted, he's jaded. But he's also one of the highest-grossing screenwriters in the history of the medium.
The simple fact is nobody listens to the writer. If the actor decides to change lines on set, they do it. If the director decides to change dialog on the fly, they do it. I have a friend who texted me from Paris saying that they had until they landed in Romania to cut 30 pages out of the script. He'd never even met the writer. I have another friend that couldn't come up with dialog on the spot when the director called him from Thailand at 3am local time so the task of rewriting fell to a local PA. I have another friend who routinely draws things that his director asks him to draw, then those things are given to the CGI people, then they're given to the writer to figure out how to make it work. And I have another friend who was flown out to rewrite some scenes on Pacific Rim while it was actually filming, even though he'd yet to have a screen credit, and they ended up using one line of dialog. From about four different friends I've heard the stories of how Producer X decides he's going to do a movie on, say, Teletubbies, so he calls in the top 80 screenwriters for pitch meetings, and he has his assistants take notes while he surfs Facebook or whatever, and then they put together their list of the best Teletubbies ideas out of those 80 pitch meetings and then they pay the assistant union scale to write it.
And he's psyched to do it.
Because now he's got health insurance.
And after his agent takes 15% off the top, and his manager takes 20%, and his lawyer takes 20%, he's making about $60k. Which is the first income he's had above minimum wage, which it took him 3 years to start making, and which he'll never see again because the next time the process is run, he won't be the assistant anymore. He'll be too expensive.
Theoretically, you can blame "the writing." Practically, nobody writes movies anymore. They're mad-libbed by committee on the fly.