Why is the "movie" becoming more and more important to Steve

The movie becomes more important to Steve because it represents its fears and the rationalization of it being a fantasy allows him to overcome them.

Explanation:

The reason behind this answer is that in the ned, Steve is very scared and wants to overcome his fears. However, he can face them because they are not real. Thus, he needs the movie because he can fight them and don't fail against them. He wants to grow, to overcome them and that is why he needs it. It is a mechanism for him to grow, he knows it and he pushes himself to do it.