Fedor does not provide an image for every DE/wm. Anyone can create a custom atomic fedora variant with the desired de.
Moreover, you can create your own image and deploy it to 100 machines and all of them will have the exact same os and packages. This may not be useful for you as an end user directly but the dev who is developing the image for you (e.g. fedora in the case of silverblue) knows that you have the exact same comouter as him and if it doesn't work on your pc, it doesn't work on his, because it is the same. Hence, better support for you.
Coming from traditional distro (Arch to be specific) I just install it without DE or uninstall the existing one and install the other. Graphical environments are just programs just like any other.
So those images are just a convenience thing? Like Fedora has spins that preinstall desktops to have them out-of-the-box?
How those distro are displayed in (neo)fetch like programs, are they just Fedora or their own thing?
Graphical environments are just programs just like any other.
They are in Fedora, too. It’s just that installing one DE overtop another can cause config file clashes (ie installing Plasma alongside GNOME means GTK apps will have a minimize button when logged into GNOME)