Why show a boundary south of Carthage?

Why draw a political boundary south of Carthage if the Sahara was already there?

The map also distinguishes Carthage from the Numidians, a militarily capable neighbouring people known especially for cavalry.

The boundary also marks the presence of the Numidians beyond Carthaginian territory. Their military capability, particularly their cavalry, matters to the political geography. A map of the region therefore has to represent neighbouring peoples as well as the desert.