Why was the Chinese state called the middle state?

In ancient times, China was a small kingdom surrounded by hostile tribes. In the north, there were endless steppes inhabited by the Huns. In the west, there were mountain plateaus where the ancestors of the Tibetans lived. Impenetrable rainforests existed in the south at that time. And in the east were the swampy lowlands of the Yellow Sea, to which the Chinese had no access. The Chinese rulers believed that they lived in the center of the world and called their country the “Middle State”.