Because apparently they were not fully human.. but inbred monsters.
From Wikipedia:
Anak is a figure in the Hebrew Bible in the conquest of Canaan by the Israelites who, according to the Book of Numbers, was a forefather of the Anakim (Heb. Anakim) who have been considered "strong and tall," they were also said to have been a mixed race of giant people, descendants of the Nephilim (Numbers 13:33). The use of the word "nephilim" in this verse describes a crossbreed of God's sons and the daughters of man, as cited in (Genesis 6:1-2) and (Genesis 6:4). The text states that Anak was a Rephaite (Deuteronomy 2:11) and a son of Arba (Joshua 15:13). Etymologically, Anak means long neck.
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