If the “Default Template” option is selected from the Template dropdown, then WordPress will set the value of the _wp_page_template meta to default and not page.php, so that’s most likely the reason why your meta box is not showing/added.

So I would use if ( in_array( $pageTemplate, array( 'default', 'page.php' ) ) ) instead of if ( $pageTemplate == 'page.php' ).

Additionally, I would also use the $post variable that’s available as the second parameter for the meta box function:

add_action('add_meta_boxes', 'page_add_meta_boxes', 10, 2); // require the 2nd parameter
function page_add_meta_boxes($post_type, $post) { // and define it in the function head
    //global $post; // <- no need to do this

    // your code
}

Update

If the meta value is empty (''), then the template will be the default, e.g. page.php for the page post type.

So you could use if ( ! $pageTemplate || in_array( $pageTemplate, array( 'default', 'page.php' ) ) ) instead of the one I suggested above.

Or actually, you could use get_page_template_slug() which is much easier. 🙂 Example based on your updated code:

function page_add_meta_boxes($post_type, $post) {
  $pageTemplate = get_page_template_slug( $post );
  if(! $pageTemplate || 'page.php' === $pageTemplate) {
   add_meta_box( 'page-repeatable-fields', 'Index', 'page_repeatable_meta_box_display', 'page', 'normal', 'low');
 }
}