Because there is no formula, recipe or related definition to make a great movie!

Its actually really rare to make a great movie. It's biggest contributor in outcome is good director and it's team but still sometimes they can produce bad or not so bad movies. In other case I think like %99 of the movies are never get close to be great, it's rare like startups.

Look at Tom Hooper, he had good movies and in the end jumped into lowest possible level.