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.