See this blog post by Christian Schaller that explains more in details why GNOME is used by Fedora Workstation and thus Fedora Silverblue.
Also, in the early days, Fedora Silverblue was not able to add Flatpaks by default during the installation, so a basic set of applications had to be installed to make the desktop usable.
Now that all basic GNOME applications are installed as Flatpak, keeping a small set of really essential applications (browser, file manager, terminal) in the base image makes sense to make sure that you have a backup plan if you remove all your Flatpak apps (intentionally or by mistake).
If disk space is the concern, you can take a look at this thread if you want unofficial non-GNOME variants.