IIRC, the word Rav appears in the Tanakh as an adjective ("עצום ורב" or "רב לכם בני לוי". Probably in Jonah 1:6 "רב החובל" it is a name?).
Earlier leaders (Moses, Yehoshua, judges, prophets) and earlier Rabbis (Zugot) weren't called "Rav". All of sudden "Rabbis" appeared in different forms: Rabban, Rabbi, Rav, Rebbi, etc.
Why after Hillel and Shammai, did the tradition break, and the title "Rav" appear?