Why Not?

Here'e what I don't quite understand.  Let us say a bank, or a Wall Street firm really is "too big to fail", why does that mean its leaders, its principals, are immunized from prosecution?

The bank can remain whole  --  just under new leadership.  In fact, a temporary "nationalization" should be in order until the "books are uncooked".

Can anyone give me a good reason why this cannot, or should not happen?