Because it would be stolen by the government. Ask people in post-Soviet countries about how Soviet governments were stealing money from their bank accounts, from all citizens collectively. No one believed in money under socialism in the USSR. The government could always pull a funny trick called "and now... it's gone". Be happy, comrade, that only your money is gone, you could also be in prison.
USSR operated under state capitalism, nothing like what any of the communists or socialists ever imagined. But I guess that is what USSR made socialism mean as a result.
BTW: Most of the world lives under state capitalism, not capitalism, not socialism, not any other form of economic system. The state has an unrestricted claim to any and all of your wealth.
