Why doesn’t flush_rewrite_rules run on plugin activation?

I’m trying to flush rewrite rules in a plugin and it isn’t working. I have to go to permalinks -> save in the admin panel for the rewrites to flush. Can anyone see why?

class SpektrixPlugin {

public function __construct(){
    add_filter('query_vars', array($this, 'add_query_vars'));
    add_action('init', array($this, 'add_rewrite_rules'));
}

public function activate() {

    flush_rewrite_rules();
}

public function deactivate() {

    flush_rewrite_rules();
}

public function add_query_vars($vars) {
    $vars[] = "event"; // name of the var as seen in the URL
    return $vars;
}

public function add_rewrite_rules() {
    $page = get_page_by_title( 'Event' );
    add_rewrite_rule( 'whats-on/(.+)/?$' , 'index.php?page_id='.$page->ID.'&event=$matches[1]' , 'top' );
}


}

$SpektrixEvents = new SpektrixPlugin;

register_activation_hook( __FILE__, array($SpektrixEvents, 'activate') );
register_deactivation_hook( __FILE__, array($SpektrixEvents, 'deactivate') );