First, banks and large international corporations are essentially the same. Some corporations even set up their own banks (though the connection may be not open, of course), so they can't possibly steal their own money. Further, you erroneously assume that the wealth of both big and small corporations consists in money they keep in banks. This is obviously not so. Even the largest corporations may have a very small amount of money in their bank accounts, just enough for current operational expenses. And if these expenses are small it doesn't make sense to keep plenty of money in the bank account
#1 No one said banks and corporations are the same.
#2 Corporations do not set up banks.
#3 Corporations do keep their money in banks. Specifically banks which function as tax havens.
#4 Many large corporations have $20-$50 billion usd stockpiled. That money has to be stored somewhere & banks is where they store it.