Apples and oranges
Brains don't work digitally, or even linearly. It's almost impossible to compare the two types of processing in any meaningful way, and it's kind of silly to try.
And such computing power is far from a boondoggle. At the very least, such computers allow the design and analysis of nuclear weapons without having to perform actual tests, which should make the environmentalists happy. Such computers also have applications in climate calculations, aerodynamic simulations, and many others.