As many famous screenwriters have been at pains to point out, a movie is a very, very large collaborative enterprise by at least a dozen departments full of people, the leader of any one of which can make or break the results.
What this means is: There are a lot of bad movies made from good scripts. Don't assume that the quality of the average finished film is representative of the quality of the average script, let alone of the skill of the average writer (who is, after all, operating under many, many constraints).
That's nothing new. Read what Heinlein has to say about the making of Destination Moon in 1949 in his Guest of Honor Speech at the Rio de Janeiro Movie Festival twenty years later. It was published in the posthumous tribute collection, Requiem.
What this means is: There are a lot of bad movies made from good scripts. Don't assume that the quality of the average finished film is representative of the quality of the average script, let alone of the skill of the average writer (who is, after all, operating under many, many constraints).