Why is it difficult to date the first sound shift that produced Proto-Germanic?

It is difficult to date the first sound shift, also known as Grimm's law, because there are no written records of the parent language, Proto-Germanic, from which the Germanic languages evolved.1

Because the parent language was never recorded, scholars must rely on the comparative method to reconstruct its structure and sound systems by comparing later Germanic languages with one another and with other Indo-European languages. While this method allows linguists to deduce how Proto-Germanic sounded and to identify systematic correspondences—such as the shift of original voiceless stops to fricatives—it does not provide a precise chronological anchor for when these changes occurred. Consequently, the first sound shift remains a reconstructed linguistic development rather than a documented historical event.123