Remove the Media Library Tab

I’ll leave the ‘why’ to you, but here’s the ‘how’

add_filter( 'media_upload_tabs', 'no_media_library_tab' );
function no_media_library_tab( $tabs ) {
    unset($tabs['library']);
    return $tabs;
}

Okay, one possible reason would be that you want to restrict users from accessing media that’s been attached to another post.