Nothing in their environment has made intelligence worth the candle. Brains require a lot more metabolic input that brawn. Survival in the wild requires optimizing cost-benefit tradeoffs, so brains never get much of a chance.
SO – why did hominids get brainy?
Two reinforcing factors.
First, before brains were very big, some hominid ancestor learned to cook food over whatever fire there was; lightning strikes, that sort of thing. That food was more easily digested, so getting more nutrients was simple. After that, maintaining fire paid off. The access to better food meant a metabolic boost, one effect of which was to reduce the relative cost of a bigger brain.
Second, over the past two million years the area in northern Africa where hominids were prominent underwent a series of about ten violent weather changes. New predators, new prey, different food plants, different strategies for staying safe and well fed, different strategies for surviving the annual period where food was hard to find. Ten of these, about every 200 thousand years. Each one favored cleverness and mental flexibility. Since brains weren’t as expensive as before, they could grow. So they did.
The above is a layman’s recollection from reading and reflecting. The real reason, though, that homo sapiens managed to grow a bigger brain was that it *could*.
Fact one: there is no “theory” of evolution, but there is massive evidence that it has been happening, starting at least 3.6 billion years ago.
Fact two: the fellow who had the nerve to state in public that new species rise up on a continuing basis was Charles Darwin. In his day it may have been called a “theory” but, in today’s dictionary, the correct term is conjecture. References to “Darwinian evolution” miss the point completely. There is the fellow who first put the idea on public view, and there is the current sun-bright understanding of its history and the life-mechanisms, e.g. ever-changing DNA, that enable it.
Fact three: “synthetic” evolution imagines a future in which computer-aided researchers learn enough to the intricacies and hidden gotcha’s of DNA to begin writing their own species from scratch. DATAPOINT: the human genome contains more that 6 billion nucleotides, hence more than 2 billion codons. (A codon is a triplet of the letters A, C, G, and T, which provide 64 possible combinations.)
Skipping the math, the human genome compares directly, in terms of information content, with two thousand beach-read novels, each with somewhere in excess of 150,000 words. TWO THOUSAND! In other words humans must use enough computer power to write a couple of thousand beach-read novels. Trust me, the actual beach-read novels are a far simpler endeavor.