No, we don't know why. You also have little basis to claim the biological visual cortex is optimal. It certainly works, which is enough for people who draw inspiration from it.
that is my point - we have very good suspicion and experimental data (successful deep learning CNN as well as visual cortex) that it is optimal, at least in some very wide class if not global, and that warrants investigation for a proof of it. Proven optimality would be very telling, especially for biological visual cortex. Even if optimality doesn't happen for CNN Gabor, there may happen to be discovered reasons for why not, and thus that would help to construct even better, probably optimal, approach.
Suspicion and empirical evidence cannot prove something is optimal. You cannot exhaustively empirically search an infinite space of models. You are seriously misunderstanding the definition of "optimal."
