The following are just my opinions, and I can't prove any of it, but...

I think "they" (US Treasury and Bureau of Engraving and Printing) have to know that, no matter what they do to the bills, a sufficiently motivated, equipped, and funded counterfeiter will eventually be able to duplicate the bills. However, by making it as difficult as possible, they raise the cost enough to make it uneconomical to do it in small quantities, and the more bills that someone makes, the easier it becomes to track them to their source and shut them down. Infiltrating a suitcase full of fake $100's into the cashflow stream is one thing, but moving three semi-loads of them is an entirely different matter.

I also believe that many countries around the world, probably including the US, manufacture counterfeit currencies of foreign countries as a form of economic warfare. A flood of counterfeit currency on the market diminishes the public's faith in the real currency, lowering its value, since the whole concept of modern paper currency is founded on nothing more than faith and trust in the first place. I'm almost positive that I've read the Bureau of Engraving and Printing already manufactures foreign currencies under contract anyway, so it would seem to be a fairly trivial step to crank out a few extras for later use, should the need for them ever arise.

As I said in the beginning, I can't prove any of this, so I might be completely full of shit. Wouldn't be the first time.