It is related to HTML semantics, an IMG is a semantically important element of the page while a background image has no semantic importance. Because IMG is semantically important, but might fail to load or render, the HTML standard requires an alternative text, a description of the image content, to be supplied with the image in order to be able to display it instead of the image.
The mainstream perception right now is that header images are just decoration and they are not part of the content, and that is why they are used as a CSS property and not with an IMG tag.
technically it should not make much difference in the code as the difference in terms of development effort between
<div><img src="https://wordpress.stackexchange.com/questions/163753/<?php >echo get_header_image()?"></div>
and
<div style="backround:url(<?php >echo get_header_image()?>)"></div>
is almost zero.
If you want to cover all possibilities and leave the most options to your users, you can let them decide by themselves if an header image is decoration or content maybe by giving them an option to add a descriptive text to the image and then if supplied use an IMG tag instead of css rules. Some SEO experts might appreciate it but it will probably fly over most peoples head.