Part of it is that we're taking Roman (by which I mean, mainstream Roman citizens and writers) at their word with the language they used to describe these peoples. The words they often used were barbaros and peregrini, the latter meaning "foreigner" or perhaps "migrant" (it is the root of the English word "pilgrim" today), and the former meaning "babbling peoples." Both terms meant essentially the same thing: A person who is not a Roman citizen, and thus someone who is less civilized.

Historians of the Roman Empire/Late Antiquity/the Early Middle Ages often have to rely on accounts, ethnographies and other sources about the people of the Migration period as written by Romans. You mentioned that historians knew that these tribes had specific names and unique cultures, and that’s partly true. It would be more accurate to say that historians know the names that our predominantly Roman written records give these people. As a collective as well, “peregrini” and “barbaros” are the terms they used and they are the words that historians have to work with for their own source material (they might also call them “paganos,” but that has obviously developed some religious connotations that may not be present in some accounts).

As for why historians/archeologists still use the term”barbarians” for the collective group of migrants in the late Roman period...mainly it is because it is useful, people understand what it means, and historiographically, it is what preeminent historians of Late Antiquity (e.g., Patrick Geary, Peter Brown) continue to call them. There is a nascent trend of historians using alternatives to “barbarians.” Ralph Mathesen, for example, has decided to start referring to them as “peregrini” instead, as this lacks the large negative connotation that “barbarians” obviously has. Even then, you will still see historians who are trying to buck this trend continue to use the term, because it is what historians use. This is a pretty typical problem for problematic language in most historical periods!

N.B.: I am an early modernist, not a classicist. As such the best book I can recommend that gets at the heart of the historical roots of the term “barbaros” is by an early modernist: Anthony Pagden’s Fall of Natural Man: The American Indian and the Origins of Comparative Ethnography.

If anyone has a more in-depth grasp of the historiographical stakes of these terms, or could provide a corrective to what I’ve written here, I would happily be educated on it!