It’s because gnome-software can’t associate the installed metainfo file with a package name, and hence it never gets adopted by gnome-software’s PackageKit plugin. (gs_app_get_bundle_kind (app) == AS_BUNDLE_KIND_UNKNOWN rather than AS_BUNDLE_KIND_PACKAGE for it in gs_plugin_adopt_app() in the packagekit plugin.)
Normally, if your RPM was installed from a repository, this link would be provided by the Appstream catalog data, which would contain a <bundle> or <pkgname> tag for your component which links it to the package name. Catalog data is different from component metainfo files.
Arguably dnf should build a local Appstream catalog when installing stand-alone RPM files. However, stand-alone RPM files are discouraged (everyone should be using package repositories) so I can’t imagine anyone being enthusiastic to implement that.
You could try adding <bundle> or <pkgname> tags to your metainfo file. gnome-software might read them, although I think that would strictly be against the Appstream spec.