It helps to know why characters do things. What characters do is one thing, but an understanding of their reasons can help make sense of things. Unfortunately, writers of drama (cinema, tv, stage) can't deal with characters motivations as clearly and elegantly as prose writers.

Motivations and thoughts are near instantaneous in our heads but to explain them takes an inordinate amount of space. Also it's best told in long passages rather than pictures or short bits of dialogue. Writing a novel meets much of the criteria to properly convey character motivations.

Actions May Not Make Desire/Reasons Why Clear

In every medium things happen, people do things. A man who never moves out of his mom's house might look like a pathetic person from the outside looking in. If we knew it was because of guilt over him pushing her down the stairs a few years before that he stays, it changes our perception. This is something that would be better told in depth in a novel rather than through voice-over narration or explanatory dialogue.

There Was A Conflict Of Ideas First

Drama is about conflict and the playing out of it between two opposing forces. This isn't the only conflict that can exist in stories. Before any action is taken a decision has to be made, a choosing of the best among many options. Along with this choosing is also a process of analysis, compare and contrast, and prognostication. These are things you cannot see or hear for they go on in one's mind, but they can be known through use of words on paper.

Outcomes Are More Clear In Prose

Before a character takes one action or another we may not be fully informed as to the why of it. In fact we may have no clue as to why one action or another was taken. After an action, in prose, we can learn a lot about the reasoning behind it all. In drama the outcomes of an action are either visual or auditory; we see the aftermath or hear how people talk about it afterward. The true measure of something is how people feel about it and this is where novels shine. We may not know why he did something but we may learn that he is pleased by what happened and given further information as to consequences.

Movie Only 1/5 Of A Book

A movie is much shorter than a book and shouldn't try to compete with it on things it does better. Even if a movie were to attempt to incorporate prose from a novel it would only be capable of doing so in voice-over or titles. Books can take 10 hours or more to finish, so even filling a film with voice-over would only reach 60 pages of a 300 page novel.

We Lie To Each Other In Conversation

There are the reasons we tell people. And the real reasons. In dialogue with other characters you can learn a lot about characters, but not everything. We don't tell our deepest feelings or desires. Characters use dialogue to achieve ends, not explain themselves. In a novel the character can speak to him or herself and be much more forthcoming. Even if the character lies, we still learn more than we would have through dialogue alone.

It's easy to want to just write screenplays because they are the basis for movies, the most popular medium of the day. You may have become a fan of theater early in life and feel such a strong affinity for it you feel compelled to write for it. All the media have strengths and weaknesses, so you can either write everything in one medium poorly or write well by choosing the right medium for your material. If it's important to know reasons why a character does something then a novel will be your best choice.