Why do Xhosa and Zulu contain click sounds?

The presence of click sounds in Xhosa and Zulu is not an original feature of these languages. Instead, these Bantu languages incorporated click consonants through long-term linguistic and cultural contact with neighboring Khoisan languages.123

Specifically, historical intermarriage between the Nguni-speaking peoples and the earlier Khoisan-speaking populations of southern Africa facilitated the absorption of these unique phonemes into the Nguni sound system. While Khoisan languages typically utilize a more extensive system of clicks, Xhosa and Zulu have integrated three of these sounds into their own phonetic inventories. This process of borrowing demonstrates the powerful linguistic influence that Khoisan languages exerted on the dominant Bantu languages of the region before many Khoisan languages declined or disappeared.4256