That's because you treat these tags as equal.

For example, in the body, if you have two unordered lists, your HTML will be something like this.

<ul>
  <li>List Item #1</li>
  <li>List Item #2</li>
</ul>

<ul>
  <li>List Item #3</li>
  <li>List Item #4</li>
</ul>

You don't want to indent your second unordered list because you know these tags or lists are treated as equal, so it's easier for you to read. If you indent it, you will believe for a second, that this UL is inside the other one, and that's not truth :P

Actually they should be from the concept of things, but a lot of people don't do it because it is a little bit unnecessary. You do not really have to do the indentation at all, even though it would be really hard to read the code it will still work.