Jewish names WHY?

Question

In the earliest torah days, a Hebrew given name was the exclusive possession of the person named, and these names were not repeated for a thousand years or more. Thus, the names of the Patriarchs Avraham, Yitzchak, and Yaakov were not re-used for over a thousand years, and the name of King David was not re-used for 1500 years (in the Gaonic period). Before the Babylonian Exile, there is no record of a Jewish child having been named after a parent or grandparent. The impression is that no such custom existed. What happened that we name after grandparents now?