We don't want this either :) Let's not boil the oceans.
Nix and Guix are both a hybrid of build-from-source and binary package management systems. The package recipes and management tools describe how to build some software from source, and then transparently provide binaries for those recipes when the binary is available from a user-approved provider of the binaries.
But, by providing the full build recipes, all the way down to the bootstrap, users can easily rebuild binaries from source to check that their selected binary provider is honest, or to find reproducibility bugs.
In the case of a package recipe for some software that does not have source code available (a binary "blob"), the recipe would just download the blob from somewhere. We don't package those things in Guix, whereas Nix does. Although, it's not hard to make custom recipes that would do that, if you wanted to.