Why some HTML5 tags must have a starting and ending tag instead of self closing them with />?

Question

In HTML5 there are tags that must have a starting and an ending tag even when their content is empty:

<script></script>   <!-- Right way. -->
<div></div>         <!-- Right way. -->
<content></content> <!-- Right way. -->

<script/>  <!-- Wrong way. -->
<div/>     <!-- Wrong way. -->
<content/> <!-- Wrong way. -->

In XML this difference doesn't exist: <node/> and <node></node> are the same entity.

Why tags like script, div and content can't be defined simply as: <script/>, <div/> and <content/>?

Answer 1

From the w3c:

A Self-closing tag is a special form of start tag with a slash immediately before the closing right angle bracket. These indicate that the element is to be closed immediately, and has no content. Where this syntax is permitted and used, the end tag must be omitted. In HTML, the use of this syntax is restricted to void elements and foreign elements. If it is used for other elements, it is treated as a start tag. In XHTML, it is possible for any element to use this syntax. But note that it is only conforming for elements with content models that permit them to be empty.

The examples you listed would generally contain content, JavaScript, or other elements, so having a proper start and end tag would delimit the scope of those elements/tags.

Answer 2

Simply because they are basically some "container" for other elements.

There are elements which are didn't used as parent elements for others, like img or base for example, this one can be closed without an closing tag with a trailing />, but it is not necessary.