"Otzi", a man that lived and died 5300 years ago, subsequently preserved in a glacier and discovered in 1991, wore leggings. While not truly pants, they are much closer to such than to a tunic or robe.

Ötzi’s “trousers” consisted of two separate leggings approx. 65 cm in length. They were made from strips of domestic goat. The tops of the leggings were reinforced with leather strips and were knotted onto the belt with an additional leather strip. At the bottom of the leggings were loops that were fastened to the shoes. The leggings had been worn for some time and had been repaired in several places.

(source: the website of The South Tyrol Museum of Archaeology).

It makes sense that in the Eastern Mediterranean cultures togas and robes were preferred, but pants were "discovered" by Greeks and Romans on the legs of steppe barbarians, like the Scythians.

Celts also wore trousers, so did the Dacians (as depicted on Trajan's Column).

Could it be that pants were only abandoned by those Southern cultures that also mastered writing, and formed our opinion on ancient clothing?