Why did humans decide to settle down in very harsh environments like Siberia or the Saharan Desert, why not live in places more moderate and more accepting to life and civilization?
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Humans settle everywhere they can. As a population stabalizes and begins to grow, it is forced to expand its territory to support its resources needs. Once the immediate surroundings can no longer support the growing population, groups are forced to leave and find other habitable lands.
When given enough time, every settlement will reach a population limit, the max amount of people that can survive on x amount of land given the current technology. Resource rich areas that have harbored large scale civilizations since ancient times have had plenty of time to reach their population limit, resulting in many periods of exploration into the surrounding areas. Because of this, you can expect an extensive spread of settlements in less resource rich lands that surround those rich lands.
There are limiting factors to this spread. For example in the Amazon rain forest, it is difficult to develop food systems that support dense nuclear settlements, and it is similarly difficult to navigate the terrain to explore. Furthermore, there is such a density of resources that one would not have to go far to find a suitable area to support a small hunter gatherer population. In the Amazon, we see a large number of dispersed independent tribal peoples and lots of uninhabited land, despite the resource density of the jungle. Compare this to some deserts which, while harsh, are located with proximity to large population centers, are navigable, and may provide dispersed pastor for animal agriculture cultivation. Even in the extreme examples you chose, you can get an idea of why you might find people living in/around those areas.
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