It’s like the Etruscan language, we can read they wrote but we can’t understand what they have inscribed. It’s very hard to understand when they aren’t other languages out there that are remnants of its older ancestor. Linguists struggled with ancient Egyptian, by the way the earliest Egyptologists were Arabs like Ibn Wa7shiyyah. Until Champollion discovered by using Coptic and the Rosetta Stone they can fully reconstruct the phonology and what it somewhat says.
As for Akkadian, Sumerian and Elamite we have to thank Cyrus’s cylinder, Old Persian gave way to understanding cuneiform. The way they deciphered cuneiform was actually amazing.