Of course it's related to the format. Many formats, like PNG and Gemtext, are inherently self-contained, while other formats, like HTML and SVG, are not.
Annoyingly, EPUB isn't inherently self-contained [1], but external access is explicitly optional [2], and it does make self-containment easier, because you can reuse resources across multiple pages, whereas HTML requires you to either duplicate the resources across multiple pages, or you have to build your thing as a single massive HTML page.
You can mandate them to be if you are building something that uses them. If you don't have control, than its up to someone else to decide, and people wanted to be able to link remote resources.
Annoyingly, EPUB isn't inherently self-contained [1], but external access is explicitly optional [2], and it does make self-containment easier, because you can reuse resources across multiple pages, whereas HTML requires you to either duplicate the resources across multiple pages, or you have to build your thing as a single massive HTML page.
[1]: https://www.w3.org/TR/epub/#sec-resource-locations
[2]: https://www.w3.org/TR/epub-rs/#sec-epub-rs-network-access