Why are overcoats used on aluminum if aluminum oxide forms on surface anyway?
Sapphire is an aluminum oxide which is transparent when in crystalline form. However when aluminum metal corrodes the aluminum oxide takes up more volume that the aluminum metal from which it was formed. So the aluminum oxide can't form the same crystalline structure as aluminum metal. Thus instead of one nice nice uniform crystalline sheet of aluminum oxide the oxide forms small crystals. Such an oxide coating reduces the reflectivity of the mirror. (Thinking of it like dropping table salt on a mirror.)

The overcoating materials keep oxygen away from the aluminum metal so that the oxide coating doesn't form.