Banks are geared to infinity.
What they loan, they don't have as assets.
What they invest, they don't have as assets.
Basically, they loan out every asset they have, then borrow more to:
a) either loan again
or
b) invest on behalf of the bank
Bank operations depend on that revolving line of credit from other, more cash rich banks/institutions/funds, to maintain their day-to-day operations, cash flow, payments, market positions, hedge exposure etc.
Basically, banks are wayyyyy too leveraged and need that money to ensure they don't blow up (or take a loss, or maximize profits).
Thats my understanding, anyway.
Could be wrong.