The Septuagint was produced at Alexandria, Egypt, where 2/5 of the population was Jewish. Since Koine Greek was the lingua franca of that part of the world at the time, Jews were losing fluency in Hebrew as they acquired fluency in Greek, especially at Alexandria, but elsewhere, too. Hebrew was still spoken, but differed somewhat from the earlier Biblical Hebrew, which also used an earlier, different form of writing. There were less people among the general population able to read the older Torah (aka Pentateuch, the first five books). Translation facilitated the ability of Alexandrian Jews to participate in reading and discussing the Torah. Only the Torah is considered sacred law, and is essential for any Jewish congregation to have. It would naturally be the first part of a Jewish community’s translation. The books of the prophets are included in the Septuagint, having been translated in following years, after the Torah. The Septuagint spread to Judea, too, where Koine Greek was also becoming the commonly spoken language, although Aramaic and Hebrew still were in use.The "facts" are that we have no record of any biblical writings prior to the Greek "translation" called the Septuagint c 280 BC. IT contains only the first five books, which indicates that the Greeks were not aware of the rest of it in the early 3d century.
Instead of Greek culture being influential in Jewish religious development, the reverse seems to be the case. Besides Hellenized Jews, there were also Judaicized Greek intellectuals who admired Jewish theological and philosophical debates over morality, interpretations of the Torah’s instructions, arguments for monotheism, etc. Once translated into Greek, the Septuagint allowed the Jewish messianic prophecies to become widespread in the Hellenized world, picked up later by Romans, and early Christians. It forms the foundation of the OT for eastern Christianity.
I don’t think the fact that there aren’t any existing older versions in Hebrew has any special significance regarding the origins of the Torah and later OT books. A few factors easily account for that. One is that not only Hebrew, but other related Canaanite languages were beginning to fade in the region, being replaced by Aramaic and Koine Greek. Add to that the Jewish rebellions against Hellenistic rule and the Jewish civil wars, followed by the destruction of Jerusalem by the Romans and exile of Jews, and it would be surprising if the older Hebrew version did survive.
The portions of 1st century BC Jewish Biblical books found at Qumran in Hebrew were written in Hebrew, not translations from Greek. They suggest that an older Hebrew version did exist, from which they were taken.
Linguistic studies of the Septuagint show that it was a translation and not an original Greek work. The Greek syntax is awkward, but matches the syntactical patterns of a Hebrew to Greek translation. The later books, e.g. Macabees, were composed in Greek.
The oldest existing Hebrew version now is the Masoretic Text, from about the 9th or 10th century AD. It became the preferred version for Jews because of discrepancies between the Masoretic Text and the Septuagint. The Septuagint translators sometimes translated idioms literally and sometimes were much freer, to the point of altering meanings.