Why is save_post hook not running?

Try this in your theme’s functions.php file, or a .php file of a plugin that you may be writing:

add_action('save_post', 'xxx_save_meta_box');
function xxx_meta_box_callback() {
  add_meta_box('xxx-meta','xxx Details','xxx_meta_box','xxx-post-type','side','default');
  error_log('meta box cb');
}
function xxx_save_meta_box($post_id, $post) {
  error_log('running ...');
  die('OK!!!');
}