Plugin not creating table on activation

My plugin doesn’t create table on activation SQL code is right and other parts of code seems right too. Why It doesn’t work ?

class MySimpleForm
{		

	public function __construct()
	{
		/*** Includes ***/
		include_once plugin_dir_path(__FILE__).'/db.php';
		/*** Instanciation ***/		

		register_activation_hook(__FILE__, array('MySimpleForm','install'));
	}

	public static function install()
	{
		MSF_db::createTable();
	}
}

new MySimpleForm();

db.php file

class MSF_db
{
	public function __construct()
	{

	}

	public static function createTable()
	{
		global $wpdb;
		$wpdb->query('CREATE TABLE IF NOT EXISTS {$wpdb->prefix}msf_form_data
			     (id INT PRIMARY KEY,
			     firstname VARCHAR(50) NOT NULL,
			     lastname VARCHAR(50) NOT NULL,
			     email VARCHAR(200) NOT NULL);');
	}

	public static function dropTable()
	{

	}
}