I'm just guessing here, but my assumption would be because those terms are ambiguous.
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First Cousins: Share a grandparent.
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Second Cousins: Share a great-grandparent.
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First Cousin Once Removed: Your first cousin's child or your parent's first cousin.
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First Cousin Twice Removed: Your first cousin's grandchild or your grandparent's first cousin.
Also, these terms are not as common in non-English speaking countries. But, again, this is just my guess.
