Maybe the alternative to buying government bonds is putting the money in an account at the central bank, which has interest even more negative? Here is the ECB addressing the question of why a bank would be willing to pay interest to deposit at the central bank, rather than putting paper in a vault: because vaults cost money to build and operate.
Aren't the big banks publically traded and expected to grow by stock market analysts? How does that work when they get negative interest rates?
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