I'm going to give two very short, simple answers and then one somewhat more complex (but still pretty short) answer. First short answer is that we say "Kingdom of Lombardy" and "Ducky or Normandy" because that is what they called themselves. Nine times out of ten the best term to use for a particular state or social group is the one they use to refer to themselves. If we use a term like "Germanic tribe" it is because we don't actually know that term.

Second short and simple answer isn't actually short and simple for you but is for me because it is just a link to some discussion about the term "tribe" which has a pretty fraught history of usage.

The third answer relates to an old but still somewhat useful concept in anthropology of sociopolitical typology, which essentially posits that political configurations can, broadly speaking, be categorized into four types: bands, tribes, chiefdoms and states. I was trying to think of an easy way to explain the difference but to be honest I am a bit at a loss, so I will just link this handy chart (in my defense, this is generally how intro anthro textbooks do it also)[EDIT: changed to an imgur link. The original citation was "based on the typology in Elman R. Service's (1962) Primitive Social Organization: An Evolutionary Perspective" but to be perfectly honest I like most people learned the typology from introductory materials,specifically the excellent lecture series "Peoples and Cultures of the World" by Edward Fischer]. This typology is somewhat out of favor, for reasons that I think motivated your question: how on earth do you classify the Holy Roman Empire? It also implicitly promotes a linear view of human society in which people progress through different "stages" ending in the modern nation-state--actual history is rather more complex. Furthermore, actual political affiliation is often much more complex and multiple, for example one of my favorite groups n history are the Isuarians, who firmly existed within the Roman Empire but also maintained an internal political configuration that can best be described as somewhere between "tribe" and "chiefdom" on that chart. All the terms have a bit of difficulty coming down to the level of the individual.