Why have proposed links between Sino-Tibetan and North American languages not been proven?

Proposed links between Sino-Tibetan languages and North American languages, such as the Athabaskan family, have not been proven because such claims are considered beyond reach at the current state of linguistic knowledge.1

In comparative linguistics, establishing a genetic relationship requires demonstrating that structural and phonological similarities are not merely coincidental or the result of long-term contact between neighboring groups. The comparative method relies on identifying consistent sound correspondences and morphological units that can be traced back to a reconstructed common protolanguage.23

Because the Sino-Tibetan family involves significant time-depth and internal diversity, comparative phonological statements are often difficult to reduce to the concise laws necessary for proof. Consequently, broad hypotheses connecting these languages to distant families like those in North America are viewed as speculative and lack the rigorous evidence required to confirm a shared ancestry.4321