Always the Same Map

Mongolia has barely enough rainfall for grass (for the most part).

And even then, grass barely survives there in some seasons. So for people to survive, they require a diet consisting of animals that can eat grass and convert calories in grass into calories humans can use. Game and herd animals are the most effective way of surviving on the steppes.

This also made them extremely potent against agricultural civilisations they faced in sieges. If the Mongols reach your agriculture and destroyed your crops, you've basically lost. They, on the other hand, can survive off animals that survive off grass.