I wonder if the reason that Eriador is 'empty' is simply because there is no great centralising kingdom to describe and to interact with either Gondor or Arnor?
Taking the risk of using clues from our own limited histories, was the Teutoburg forest empty in the times of the Roman Empire? Obviously not, as poor old Varus (?) found out. [snip]
A lack of Historical Evidence on our part (or Tolkiens!) does not mean a lack of history. Eriador is full of people and legends and all sorts of nasty stuff. Believe me, I've been there.
For the first point, I'd rather say there is no
central authority (e.g. a kingdom) to provide the security and support necessary for growth. There are probably scattered people around (outside Shire & Bree) in that "100 league" radius, but in scattered small hamlets or farms. These are alway in danger of being wiped out be several causes - and this happens (in the OM interpretation) regularly. Therefore, these small pockets come and go, without the ability to reach a stable and lasting existence.
Regarding Teutoburg, Arminius assembled his army from all over the place, not only this small place of the Teutoburg Forest. But you are right, that the supposedly "empty" Germania was full of warlike people. OTOH the Romans were aware of these warlike people from early on (Cimbri and Teutoni!) and tried to bring them under their rule by several means (not only military). So I am not sure whether the area was considered "empty". Sparsely settled sure, but not really "empty".
It is true that Tolkien wrote little on Eriador, but he made key statements that set the larger frame: Large parts of Eriador devoid of organised (and thus lasting) settlements of men. It is the job of the later sub-creators to fill this with interpreations to make this plausible
The point that there is no organised settlement in the designated area leaves room for temporary settlements that come and go quickly - either through emigration/movement or death by several means.
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