Add_meta_box not appearing, but does appear in screen options

Question

You missed one argument in the $args for add_meta_box.

The correct use would be (Codex):

add_meta_box( $id, $title, $callback, $post_type, $context, $priority, $callback_args );

You forgot to set the context, add it and you should be fine.

add_meta_box(
    'content-on-page',
    'Content On Page',
    'ila_render_meta_box',
    'page',
    'normal', // add this line
    'high'
);