Why do we use Document object to create Element ?

I have a question about create Element , TextNode or anything else like that in DOM. 
 
Why do we use Document object to create Element ? 
For Example: TextNode el = document.createTextNode("name").And then add el to one node.
 
Why don't we design a class like TextNode ? 
And then instantiate it directly like  TextNode el = new TextNode().
Finally,use a node to add it,like node.appendChild(el).
 
 
I think that we use  Document object to create Element is unnecessary and is not fit to our thinking.
 
 
Do you have any suggestions about that ?
If you have ,Please send e-mail to yexianyi@hotmail.com
 
Thanks very much!
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