Because you’re missing the point: if you want to build offline archives you can’t use something that is online by definition.
It really baffles me how careless people are with depending on internet 24h7 these days and how it completely goes through your head that maybe, if you’ve to deploy dozen of machines a day (automated or manually) you might not want to be redownloading the same stuff every single time and maybe a few packages aren’t even available on the provided ISOs at debian.org. Additionally some people are required to work on airgapped networks and systems that can’t have Internet access to download packages…
Since you didn’t care to actually read most of the article I’ll just say this:
Larger images, such as Blu-ray, aren’t available for HTTP download however the Debian project provides Jigdo template files that can be used to build those images from the repositories. Jigdo makes it possible and easy to create offline images containing with all packages the repository has to offer.
This is just perfect, a community about self-hosting that can’t even self host the basic software they require lol