Why is the

Question

For curiosities sake, why is the <img> tag not closed in HTML?

Answer 1

It is what is called a void element which just means the element must not have any content (but can have attributes.) The HTML5 spec has this to say about void elements:

if the element is one of the void elements, or if the element is a foreign element, then there may be a single "/" (U+002F) character. This character has no effect on void elements

So there's no real reason to have the single "/" (U+002F) character, but it won't break anything if it's included.