This is called a reserve and is intended to serve as an insurance policy. The commercial banks hold their reserves with the Central Bank or with a third party institution, explicitly and exclusively set up for this purpose. The Borrower pays Bank A interest on the loan that he receives. This process is called credit multiplication. The Western Credit multiplier is 9.
Banks are the most heavily leveraged businesses in the world.
But this is only part of the problem. Another part is that the profit margins of banks are limited. We, as consumers of bank services do not think so - but banking profits are mostly optical illusions. We can safely say that banks are losing money throughout most of their existence. This spread is supposed to cover all the bank's expenses and leave its owners with a profit.