Why do desert and mountain regions have fewer people than the other regions

Because those regions have different unfit for many people climates the hot desert isn't a place for people to captivate at the mountains are higher which means less oxygen there's not many places for stable building and deserts its unsafe due to some sandstorm.sandstormetc getting to my point they are both places not many ppl live period

It may be becouse the lack of water.

In the midwest because it has more moderate climate than the north or south west which makes farming far easier.