Actually, you don't really need to address the "why" of it. Your goal of deleting everything from your iMovie Library is accomplished by Control-clicking on the Library's icon, choosing Send to Trash from the pop up menu, and then emptying the trash. The next time that you open iMovie it will open in a new blank (empty) library. Deleting the library or its contents is not recommended because you will lose all of your projects, and any media that you have not stored elsewhere other than in the iMovie library. Even the projects that you decide to keep will be disabled if you have deleted any media to which the project refers. However, that would be your call, of course.
The better way to cure space problems is by purchasing an external drive and storing your iMovie library on the external drive. The drive must first be formatted Mac OS Extended (Journaled) or APFS. It may be too late for the present library, because the deletions likely caused the library to become unworkable, depending on what procedures that you used. .
Your question of why you still have 30 GB in your iMovie library when you believe that you have deleted everything from it, would require more more details to address. What steps did you use to delete the projects and media? Did you delete them from the iMovie app or from the Show Package Contents of the library? What is shown when you Control-click on the Library's icon in the Movies folder and select Show Package Contents from the pop up menu? Is there anything in there? When you Control-click/Get Info on the library's icon, how many gigabytes are shown for the library's size?
-- Rich
-- rich