Why do linguists treat North Caucasian and South Caucasian as separate language families?

Linguists classify North Caucasian and South Caucasian as separate language families primarily because there is no conclusive evidence of a shared genetic origin between them.1

In comparative linguistics, establishing a genetic relationship requires the identification of regular sound correspondences—consistent patterns of phonetic change across languages that descend from a common ancestor. While researchers have identified internal groupings within the North Caucasian branch (comprising the Northwest Caucasian and Northeast Caucasian families) and confirmed the internal unity of the South Caucasian (Kartvelian) family through shared vocabulary, grammar, and phonology, these same systematic links do not exist between the North and South groups.123

Consequently, at the current stage of linguistic research, the interrelationships between North and South Caucasian remain uncertain. Without the demonstration of regular sound correspondences, they must be treated as distinct families rather than branches of a single linguistic lineage.1