Because other people were there.
There is a limit to how many people you can put into an area – and that limit has grown over time as better building means more dense housing, as well as better farming means more people fed on less land (and human effort). However, regardless of what that limit is, humans tend to find that limit relatively quickly, and then start fighting each other over that area. Some times, it is easier to just go somewhere else than keep fighting for the better place – which eventually drove people into even the most barely hospitable places on Earth.
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