The reason is because only the most important documents were transcribed through history. Many works might have been lost and only those which people bothered copying survived. Some of those are philosophical, most of the early Jewish contributions to philosophy came from Qaraite Jews. Rabbabinic thought placed more emphasis on legislating the tradions of second temple Judea than on philosophy until the 10th century or so.
Qaraite Jews however only had the Tanakh to work with and therefore devoted a great deal of effort to philosphy. Most of the Mutakallimun (Jewish Kalamists) were Qaraite like Ya'aqov Qirqisani, Yehuda Hadassi, Daniel Qumisi, Yefet ben Ali, Yehoshua ben Yehuda, Yosef ben Avraham, et cetera and only Saaida Gaon and Rambam contributed to world philosophy in a major way until Baruch Spinoza. Of course the Ramban and many of the Rishonim deal with philosophy indirectly through commentaries and contemplations on Hashem, prophecy, ethics and other subjects but I mean works that are soley intended to be read as philosophy like Guide for the Perplexed.
It may have been that Qaraite Jews were just more predisposed to philosophy since they originated from the Sadducees (who were accused of being secular philosphical types) and were only preoccupied with preserving the mesorah of the Tanakh in Tiberius (most or all of the Masoretes were Qaraites including Aharon ben Asher and Shlomo ben Buya'a, the Allepo Codex was actually written for the Qaraite synagouge in Jerusalem) and wandering around Israel looking for aviv barley, so they probably had a lot of time on their hands.
The Tanakh has a sort of existential bent to it which probably influenced them. Also a few philosophical works surived from the first temple period like Qoheleth and Iyyov. In addition to Philo another second temple philosophic text is the Sadducean work Ben Sira.