The final draft of a screenplay is never a transcript of the finished product.
Marvel Studio’s tactic is really just an extreme case, with millions of dollars flying around.
While Marvel’s particular approach in this department may be unique (not many movies are throwing around the same kind of money), the reality is that a vague version of this strategy has been a part of filmmaking from the very beginning.
There is an axiom in the industry that a movie is written three times: once as the script is coming together; again as new ideas are being thrown around during production; and a third time as everything is being cut together.