Why there is no tree tag in html?
There is. Nested lists (ul, or ol).
<ul>
<li>foo</li>
<li>bar</li>
<li>test
<ul>
<li>foo</li>
<li>bar</li>
</ul>
</li>
<li>example</li>
</ul>
And with the help of CSS and JS you can make the tree look and interact like, for example, the windows explorer.