Why is Gibraltar called one of the Pillars of Hercules?

Northern pillar: the Rock of Gibraltar, known in antiquity as Calpe .
Southern pillar: Abila , on the African coast, usually identified as Jebel Musa in Morocco or Monte Hacho in Ceuta.

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In classical mythology, the two great rocks on either side of the Strait of Gibraltar marked the edge of the known world and were called the Pillars of Hercules. The Rock of Gibraltar, known to the ancients as Calpe, is the northern pillar; the southern pillar is on the African coast.
Ancient Greek and Roman writers described two mountains flanking the entrance to the Mediterranean as the Pillars of Hercules, said to have been set up by Hercules during his labours. They were seen as the boundary of the known world, beyond which sailors should not venture. The northern pillar is the Rock of Gibraltar, known in antiquity as Calpe, a name still used in Gibraltar today. The southern pillar, Abila, is identified with a mountain on the North African coast, usually Jebel Musa in Morocco or Monte Hacho in Ceuta. On a clear day you can see the African coast from viewpoints such as Europa Point and the Upper Rock. The legend is reflected in symbols and monuments across the region.

Why is Gibraltar called one of the Pillars of Hercules?

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