Why is the population density different in different places on Earth?

Population density is an indicator of how many people live on a certain unit of density. The largest population density on the planet is in large cities. Thanks to the development of the construction of multi-storey buildings, many people can live here on one unit of the Earth’s area, just on different floors. At the same time, favorable living conditions have been created in the cities and everything you need is available. In some places of the planet, there are no people permanently living there at all. This could be due to harsh conditions or inability to get there. Therefore, the population density there is less.