The short answer: evolution.
Natural selection is lazy. It doesn't work to make things the best they can be. It just makes them good enough to have a decent chance of survival. It didn't make cheetahs run 70 m.p.h. just for the fun of it; it did so because that's about how fast you need to run to catch a gazelle. And gazelles only run so fast because cheetahs eat the slower ones. It's a classic arms race. The cheetah can catch its meal, but usually only by really working hard at it, and the gazelle also pretty much has to give its all to escape. Life's not easy at all for either of them; they're both working at their very peak effort just to survive.
So nature runs on the principle of "good enough" rather than "the best possible". Very rarely does nature equip some species with a trait that is far more than the job of survival calls for. The only reason pronghorn antelope run so much ridiculously faster than any North American predator is because up until a few thousand years ago, there were cheetahs here too. I don't know of any measurements that would confirm this, but I'd be willing to bet that today's population of pronghorns, without cheetahs selecting for his speed, are slower on average than their ancestors. There are more ways for mutations and genetic drift to reduce the efficiency of a runner than there are to improve it.
At first glance, we humans might appear to be an exception to this general rule. We are so much more linguistically, culturally and technologically powerful than even our closest primate relatives, it's tempting to think our relatively massive brains and corresponding intellect is a freakish anomaly. (I don't want to get into a debate about human vanity in assuming ourselves to be the smartest in the animal kingdom. I'm using "smart" in the fairly narrow technical sense of having bigger and more versatile brains, so please don't read into it any kind of value judgment. Plants are not "smarter" than we are because they "know" how to photosynthesize and we don't, and cockroaches are not "smarter" because they're more likely to survive as a species in the long run. For all their superior survival odds, cockroaches' brains are tiny and support very limited cognitive function. Plants don't even have brains. So that's all I mean by "smarter", and to argue otherwise is, well, not very smart.)
On a survival level, our brains certainly seem to be disproportionately powerful. They've made us into one of the most effective hunters on the planet, having wiped out virtually all the edible megafauna on most continents within a few centuries of our arrival. We've figured out how to produce food surpluses through agriculture, and our population has exploded to the utterly outrageous figure of 7 billion relatively large mammals. Survival for many of us, at least in the developed world, isn't even a challenge any more; most of us die from cancer and heart disease instead of starvation or being eaten by predators. This is a direct result of our species' unprecedented technological prowess.
So how does this figure into Nature's "good enough" approach? How come we got so absurdly smarter than we needed to be to survive?
A big part of it is the arms race principle. We're a social species, but not a eusocial one. That is, while we tend to live together in groups and cooperate for mutual benefit, we're not completely selfless about it the way ants, bees, some wasps, termites and naked mole rats are. We cooperate and compete with each other, and when we compete, it's usually by way of our brains. Of course, it's a lot more complicated than simple competition, and often that competition takes place within a cooperative framework. A group of hunters may be genuinely trying to cooperate to bring down a mastodon, but they may also be competing to establish social dominance. Even the fully cooperative human has to be able to detect attempts to cheat, and the would-be cheater needs to be able to figure out and defeat those detection attempts, and so on. In short, humans with bigger brains than their fellow humans were more likely to pass on their genes, and this arms race has produced a species with way more smarts than we need simply to squeeze food from our environment and avoid getting eaten by bears.
And there, in our competition with our fellow humans, nature has made us just barely good enough to have a decent chance at figuring out each other's (and even our own) motives and schemes, and not one bit better than we need to be. Sure, getting food might be relatively easy, and we don't even have to think about avoiding hungry wolves or tigers now, but the countless other struggles of social life remain as hard as they've ever been, and our brains pretty much have to work at peak capacity for that.
In fact, in many ways, our brains are facing much harder problems than they ever evolved to solve. Fact is, as big as our brains are, they're really not that good at solving certain kinds of problems. They're good at forming judgments about the kinds of things we encountered as hunter-gatherers, but they're not so good at things like formal logic and statistics. We are equipped with a whole lot of shortcuts and quick and dirty heuristics that give "good enough" results for basic survival, but aren't always the optimum or rigorously correct solution. We can learn to do calculus or apply Bayes' Theorem, but it takes a lot of effort.
So life is hard, and it pretty much always will be, because of the way we evolved and because of how evolution works in general. Our minds, our bodies, our willpower, all are the result of a process that makes things just barely good enough to survive in their environment, and not one bit better. And I'm not at all convinced that's a bad thing.