Why custom widget won’t appear in Block editor (Gutenberg)?

I created a simple custom widget by adding the following code to my functions.php file:

// Creating the widget 
class wpb_widget extends WP_Widget {
      
    function __construct() {
    parent::__construct(
      
    // Base ID of your widget
    'wpb_widget', 
      
    // Widget name will appear in UI
    __('WPBeginner Widget', 'wpb_widget_domain'), 
      
    // Widget description
    array( 'description' => __( 'Sample widget based on WPBeginner Tutorial', 'wpb_widget_domain' ), ) 
    );
    }
      
    // Creating widget front-end
      
    public function widget( $args, $instance ) {
        $title = apply_filters( 'widget_title', $instance('title') );
          
        // before and after widget arguments are defined by themes
        echo $args('before_widget');
        if ( ! empty( $title ) )
        echo $args('before_title') . $title . $args('after_title');
          
        // This is where you run the code and display the output
        echo __( 'Hello, World!', 'wpb_widget_domain' );
        echo $args('after_widget');
    }
              
    // Widget Backend 
    public function form( $instance ) {
        if ( isset( $instance( 'title' ) ) ) {
        $title = $instance( 'title' );
        }
        else {
        $title = __( 'New title', 'wpb_widget_domain' );
        }
        // Widget admin form
        ?>
        <p>
        <label for="<?php echo $this->get_field_id( 'title' ); ?>"><?php _e( 'Title:' ); ?></label> 
        <input class="widefat" id="<?php echo $this->get_field_id( 'title' ); ?>" name="<?php echo $this->get_field_name( 'title' ); ?>" type="text" value="<?php echo esc_attr( $title ); ?>" />
        </p>
        <?php 
    }
          
    // Updating widget replacing old instances with new
    public function update( $new_instance, $old_instance ) {
        $instance = array();
        $instance('title') = ( ! empty( $new_instance('title') ) ) ? strip_tags( $new_instance('title') ) : '';
        return $instance;
    }
     
    // Class wpb_widget ends here
} 
 
 
// Register and load the widget
function wpb_load_widget() {
    register_widget( 'wpb_widget' );
}
add_action( 'widgets_init', 'wpb_load_widget' );

But it won’t appear in the block editor (left pannel on the image) on the front end (see here). It only appears in the back-end, Appearance > Widgets, see here.

I am a beginner with WordPress, I looked on how to solve this problem but could not find a solution.

Best regards

Aymeric