In my words: evolution of the species is an interesting phenomenon. When a new "invention" is tested, it does not replace the original organ, so in case it doesn't hit the mark, and isn't better, it doesn't set back the entire species.
This is especially true with organs that are mandatory for life, like the brain.
According to this hypothesis (not a fact!) the human brain has three detectable stages that grew on the top of each other and are still functional, therein lies the problem.
The reptilian brain that is selfish, reactive, and knows only a few tricks, eat, shed, fight or flight.
The mammalian brain where we have our emotional (limbic) center... why? beats me... I guess mammals have emotions?
And the frontal cortex, where all higher function is performed, especially choice.
Unfortunately to us, our lowest has a tendency to run our lives...
The reptilian brain has a strong connection (maybe just seemingly) to the subconscious, or other than conscious mind...
The triune brain is a model of the evolution of the vertebrate forebrain and behavior proposed by the American physician and neuroscientist Paul D. MacLean. MacLean originally formulated his model in the 1960s and propounded it at length in his 1990 book The Triune Brain in Evolution. The triune brain consists of the reptilian complex, the paleomammalian complex (limbic system), and the neomammalian complex (neocortex), viewed as structures sequentially added to the forebrain in the course of evolution.
The triune brain hypothesis became familiar to a broad popular audience through Carl Sagan's Pulitzer prize winning 1977 book The Dragons of Eden. Though embraced by some psychiatrists and at least one leading affective neuroscience researcher, the model never won wide acceptance among comparative neurobiologists. Comparative evolutionary neuroanatomists currently regard its claims about brain evolution to be outdated.