Maps, geography and empty places in Middle-earth

I'm currently rereading the books and I'm way more interested in geography and maps now than I was 20 years back when I got into LOTR.

I'm looking up every location mentioned in the story on the map of Middle-earth as I go. I can't help but notice that a lot of the factions and cultures that feature in the story are concentrated in a handful of places while vast regions seem sparsely inhabited or entirely empty.

I'm curious to what extent this is a conscious part of Tolkien's storytelling versus neglecting to "fill in" the map in places, so to speak.

I'm not talking about places that are thinly inhabited for natural reasons -- mountains, deserts, swamps etc. Rather there are regions that contain significant and highly developed settlements or cultures within them but are otherwise presented as virtually empty:

- The Shire and Bree feature mild weather, good farmland and lush greenery but Arnor as a whole is totally abandoned? The company never seem to run into anyone in weeks of travel

- The Anduin feels like it should have major settlements all along it, yet (looking at the map) 80% of its banks seem to be uninhabited wilderness or abandoned monuments like Amon Hen, the Argonath etc. Only in the very south we have Gondor

- Other minor examples, e.g. Isengard appears to have the natural resources to build a massive army, yet the surrounding Enedwaith seems empty and underdeveloped

I'm curious if Tolkien ever talked about this or what other geography nerds thought when looking at the map!