That's because you are calling the alerts when the page loads. At that time, there is nothing yet on the textarea.
Bind the event to something that will happen after the textbox has something to show, for example, when you click the submit button:
$(document).ready(function(){
$('#editor').ckeditor();
$('input[type=submit]').on('click', function() {
window.alert($('#editor').val());
});
});
Also, you may want to bind the click event to the document
instead, so it will happen even if you add new submits programatically. For that to happen, bind the event like this:
$(document).on('click', 'input[type=submit]', function() {
window.alert($('#editor').val());
});