The answer is that the alternatives to depositing excess reserves are also costly. In fact, the cost of using alternatives to central bank deposits determines to what extent the rate on excess reserves can go negative in practice. Banks can always choose to hold physical currency instead of electronic money in their accounts with the ECB.
But Keynes coined the term “animal spirits” to describe the emotional, non-purely logical state that influences our decision-making.1 With the unusual sequence of large drops and rebounds in capital markets in recent weeks,2 it is difficult to argue that fear and animal spirits were not also at play. Obviously, given that expectations determine the evolution of the economy, they matter for monetary policy.