If having a large brain is such an advantage for humans, and it seems hard to argue considering where we've ended up in the pecking order of species, then how come more creatures don't have large brains?

This article in Discover magazine cites a few explanations from scientists:

the brain is a huge energy-consuming piece of tissue, and humans had to evolve smaller, more efficient guts to compensate for the additional energy allocation to the brain
we altered our diet to higher density energy sources like meats and seeds
genetic mutations reduced energy transport proteins to muscles in favor of those funneling energy to the brain

If you were creating species like you were creating videogame characters and had to allocate a fixed energy supply among brain and other body parts, evolution came down in favor of dedicating more to the brain when it came to humans.

One of the intriguing and repeated tropes in comic books was that a person with an abnormally large brain inevitably turns to a life of crime and is in fact often a literal criminal mastermind. It's as if the only logical outcome for such a dominant intellect is to turn to a life of crime as any other behavior -- altruism, generosity -- would be inefficient and illogical. It may also reflect some deep-seated distrust of people who are too smart.