why do I have to use URL in CSS to change SVG element code example

Example: add svg to html

<!-- external svg -->
<img src="mySvgImage.svg" />

<!-- internal svg example -->
<svg viewBox="0 0 200 200" xmlns="http://www.w3.org/2000/svg">
  <rect width="100" height="100" x="50" y="50"/>
</svg>