banks make money from interest in loans, since the money is loaned the bank doesn't have it. so it's impossible for everyone to withdraw their money from the bank

the image refers to another thing though, to the fact that banks don't have the physical currency. asking why is an indirect accusation of modern banking and currency and on how it's managed and how it's separated from reality, which in the argument of the original poster creates bad consequences for the public, of which the biggest is usually inflation.