Even the remnants we have from early languages that had a writing system are relatively  Classical Latin and Greek, Hebrew, and a few other languages are better documented than others; but for most all we have to go on are a handful of tablets here, a few inscriptions there, barely enough in many cases to allow us to identify the language, let alone draw any conclusions regarding its structure or meaning.
We can only rely on tablets, which are not enough to draw conclusions regarding language structure and meaning.