We can have $515 trillion in utterly worthless derivatives, but the heavens would fall if we spent $2 trillion per year on projects that are necessary to the survival of humanity

I think -- without having given a great deal of thought to the subject ;-) -- the difference is between a Closed and Open analytical/praxiological environment.  Derivatives are Closed under CAPM as they 'fall-out' of well understood mathematical constructs¹ developed within a generally accepted intellectual stance².  Public spending for the Public Good is Open in that fundamental questions, e.g., what is the Public Good, are uncertain, dependent on the interpreter, and controversial.  Another way to look at the difference: Closed allows one to pretend to be 100% objective while Open, by its nature, precludes that pretense.  

¹ How congruent these constructs are to Reality is another topic!

² How congruent the generally accepted intellectual stance is to Reality is another topic!  ;-)

She believed in nothing; only her skepticism kept her from being an atheist. -- Jean-Paul Sartre