It’s schizophrenia with concurrent mood disturbance. Why does it need its own code?

I just reread the DSM entry on it and I’m annoyed. They talk plenty about how to distinguish it from Schizophrenia and from mood disorders w psychotic features. But they never actually discuss what makes it unique, what challenges it presents compared to similar diagnoses, etc.

Why don’t we have a special diagnosis for PTSD with psychotic features, PTS-A Disorder? Why not OCD with mood disturbance, OC-A Disorder? Why do we get a special combo for JUST THESE TWO?