As far as installation process:
If I go to the site of any libre project that doesn't install through nix/apt/etc, it will have a focused list of directions that I need to do to install it.
If I go to Microsoft's site and search for how to install Windows, I will be greeted with a deluge of articles I need to read and understand all of the various different methods and scenarios (after avoiding the links to BUY BUY BUY. I already have plenty of Windows licenses that were anticompetitively bundled with every old laptop I have sitting around, thank you). And then since I want to avoid their consumer install methods that insist on holding victims by the nuts, I will likely need to go an eNtErPrIsE route - meaning even more reading between the lines of overwrought bullshit.
Whereas if I download a torrent of Windows, it will come with a focused list of directions that I need to do to install it.
BTW doesn't Rufus run only on Windows? That's kind of pointless for me. My workflow is virt-install --cdrom /path/to.iso
Perhaps I will look into setting up a "KMS server" next time I need to reinstall, but I would guess it's a bunch of admin work for not much gain. The kind of admin work that will have fallen apart in the few years until I need it again.
... doing a quick look it seems like "KMS server" only runs on Windows itself? And there is a libre reimplementation for Linux, but it doesn't seem to be in nixpkgs, and requires setting up a heavyweight "Domain" with Samba? A few lines in smb.conf or nixos config and I'd be game, but no, it looks just as bad as I thought it would be. Please correct me if I'm missing some way that is truly straightforward, but this doesn't seem to be the case!
So yeah in short, that's why.
There are trusted tools out there, like Rufus, that will enable workarounds for you if you tell them to create bootable media. Tools with developers you can look up, rather than anonymous pirates.