You missed the multi-page forms part. As soon as you have state of some fields affect how other fields or parts of the page display it becomes complex really quickly. There is an explosion of possible states. If you've ever seen code like this, you know why more modern js frameworks were invented:

    onSomethingChanged() {
      $('#foo').prop('disabled', true);
      $('#spinner').removeClass('invisible'); 
      $('#error-1').prop('hidden', true);
      $('.buttons').hide(); 
      if (value) {
        $('#btn-2').show();
      } else {
        $('#btn-2').hide();
        $('#btn-3').show();
      }
      // etc ...
    }