Why is reconstructing Proto-Tibeto-Burman difficult without ancient written records for most languages?

Reconstructing Proto-Tibeto-Burman (PTB) is challenging primarily because, with the exception of a few "major" and "minor" literary languages—such as Tibetan, Burmese, Tangut, and Newar—the vast majority of Tibeto-Burman languages did not leave behind written records predating the early 20th century.1

The absence of historical documentation complicates the application of the comparative method, which relies on identifying systematic sound correspondences to reconstruct a common ancestral language. Without ancient texts to provide a window into earlier stages of these languages, linguists must rely on modern data, which has been subject to significant change over time. Furthermore, the Sino-Tibetan family is characterized by a high degree of internal diversity and a long separation time between subfamilies, making it difficult to reduce phonological data to concise, regular laws. While scholars have reached a consensus on many features of PTB, the lack of deep historical attestations remains a fundamental obstacle in the field.2341