Prosecuting financial fraud is usually a great deal slower and more complex because it's more difficult to prove intent (compared to both incompetence and pursuit of legitimate profit), because it can involve so many more people (many of whom may not have been doing anything wrong at the individual level, but whose actions taken together were wrong at an institutional level), and because the rewards are more diffuse and indirect (unit profits lead to pay rises or career advancement for those involved, rather than bags of cash or deposits into secret accounts).
Signing someone else's name at the behest of a department head is pretty clear fraud (and more obvious when it's 1000's of documents being signed with someone else's name and backdated). Watch the 60 Minutes segments.
Prosecutors don't want to take on the financial sector because it ruins their achievement record if they lose or it takes too long because the defendant can afford good lawyers. They stick to the people who cannot afford a defense.
No-one has been prosecuted for the financial crisis, I mean the economy was DESTROYED, we are years into it now.
That's not to say that people can't or shouldn't be prosecuted, just that it's a more difficult undertaking. A recent example: http://www.housingwire.com/2011/04/19/ex-tbw-ceo-lee-farkas-...