Discover pretty much only shows you applications and not things like libraries or (in this case) other data that support those applications. I assume this choice was made so you don’t have to scroll through a hundred or more texlive items depend on what you search, but you’re right that not every package is included. This is an upstream KDE thing, but it’s also true for Gnome Software as well.
Upstream? So KDE decides which packages are available through Discover for every distro that uses it? That doesn’t make a lot of sense to me, given that distros decide what they package and put in their repos, and Discover is jsut a frontend, no?
Top level packages (apps and some drivers) are shown in the gui gnome-software. The underlying libraries and dependencies are not and are pulled in as needed when the app is installed. Discover does the same.
With over 3000 packages installed on my system (and at least that many additional available) a gui that displayed all would be unusable.
In contrast, I can see every individual installed package from dnf on the command line.
Where the decision is made on what to show is immaterial, merely that it makes things manageable and understandable (and reasonably sized) for the average user is the key in this discussion.
Users who want or need more detail should be comfortable with the alternate command line methods of package management.
The developers of the discover app do indeed determine what amount of info is shown to the users. It is after all a tool they develop to assist users in managing their machines. As with every app at every level the developers are the ones who design and produce it. For complaints about that decision a bug should be filed as appropriate stating the specific changes you would like to see. If they agree then those changes may appear. If they do not agree then the changes will not appear.