We intentionally made media library items not searchable because this goes against our understanding of the WordPress ethos behind how media library items are different from “content”.
For example, on our client sites, if we want to create a “Video” section, we’ll create a custom post type called “Videos”. Each Video has fields for title, description, creator, editor, etc., with a “File” box that allows a user to select a media file from the media library when they create the new Video post.
Then, using WPUS, somebody could search through Videos based on “creator”, “editor”, “category”, “tag”, etc.
However, if we search directly through the media library, you have no control over what items appear in the results. Some plugins will create several different sizes of images and store them all in the media library. When you upload a new background image using the Theme Customizer, it goes into the media library. Every image and every other piece of content you’ve uploaded to the site all gets dumped into the media library. You also don’t have the ability to (easily) add custom fields and metadata to media library items, as well as categories, tags, and custom taxonomies.
I’d strongly recommend using the media library as a dumping ground for all of your content, and then using custom post types to make them accessible to the user.