0 is cheaper than storing $1,085,500,000 U.S. dollar equivalent in euro notes. It’s also more liquid.
The Zero lower bound is below for zero for practical reasons. If the rates go too negative it becomes cheaper to just do like Scrooge McDuck and store physical cash in a Money Bin.
Zero lower bound
The Zero Lower Bound (ZLB) or Zero Nominal Lower Bound (ZNLB) is a macroeconomic problem that occurs when the short-term nominal interest rate is at or near zero, causing a liquidity trap and limiting the capacity that the central bank has to stimulate economic growth.
The root cause of the ZLB is the issuance of paper currency by governments, effectively guaranteeing a zero nominal interest rate and acting as an interest rate floor. Governments cannot encourage spending by lowering interest rates, because people would simply hold cash instead. Miles Kimball suggested that a modern economy either fully relying on electronic money or defining electronic money as the unit of account could eliminate the ZLB. Even without such measures, however, several central banks are able to reduce interest rates below zero; for example, the Czech National Bank estimates that the lower limit on its interest rate is below -1%.The problem of the ZLB returned to prominence with Japan’s experience during the 90’s, and more recently with the subprime crisis.