Wolf-
If I am not mistaken, the Rambam in his Introduction to his Peirush Hamishnayos says that the reason those tana’im and all the earlier great people were not referred to as rabbi is because they were so great they did not need a title, as their name alone elicits respect. Similar, lehavdil, to great philosophers, doctors, and artists in history who are known only by their first names.