Because the only way for a European bank to have money as 'bits in some computers' is with the ECB. And the ECB gives you back less money than you gave them (ne

Because the only way for a European bank to have money as 'bits in some computers' is with the ECB. And the ECB gives you back less money than you gave them (negative interest). Banks can also place money with other banks, but that is lending to other banks. Besides, this only shifts the problem to another bank, who now has cash on hand that'll be stored in the ECB if they don't do something else with it. Besides that, banks can invest cash. But in a low-yield market where all stable investments give very low, or even negative, interest, that becomes less attractive.