Banks do not give you private key for your own money. They can always block your account with absurd reason. This gives you stress and paper work. 

Transfering high amount of money in banking system is always problem. They ask about questions about your transfer, source of money, and many more. 

You will be treated as a criminal although you are the owner of the money. What a ridiculous situation right. 

One day you could wake up and bank would crash and you would try to save your money. If it is over limit of government warranty, you only get that limit of money. 

 There is a famous quote about banking system

"Give me a gun I will rob a bank, 

 Give me a bank I will rob the world"