Soooo... why don't the ECB lower their rates on excess reserves to zero?

They don't do it permanently, because the remuneration rate for excess reserves sets the floor under the interbank rate. Which the CB wants to do if it wants to conduct active interest rate policy. It shouldn't do that IMO (I favour a zero interest rate policy and conducting monetary policy at the exchange rate and margin requirements instead), but that's a subject for another time.

Or even a negative nominal number?

- Jake

Friends come and go. Enemies accumulate.