It’s not always about length. OP is asking about important scenes specifically being cut. One of the major reasons is because the screenwriter is most often not the author. Only in rare cases will the author participate in the screenwriting process or be the sole screenwriter. And even though an author might feel that a scene is essential, and many of their readers may as well, the screenwriter is an artist in their own right and will engage with the material differently, and understand the story differently, creating a whole new work of art with the source text. They are building a multi-sensory experience for the viewer, while the author can only try to evoke that experience through inspiring a reader’s imagination. With a new medium and new artistic personality, a new work of art gets created that can appeal to people who have not engaged with the source text at all. At the root of it is a difference of opinion in terms of what is deemed important.