"Despite continually mapping the distribution of languages across the world, scientists have few clear answers about what caused the emergence of thousands of languages. Collectively, human beings speak more than 7,000 distinct languages, and these languages are not uniformly distributed across the planet. For example, far more languages are spoken in tropical regions than in temperate areas."
https://theconversation.com/why-are-so-many-languages-spoken-in-some-places-and-so-few-in-others-116573