In civilisation there is far less impetus to use one's own intelligence to survive. In primitive society, those who best used their wits, who had energy, who coped best, would survive and live to have children. As a result of this, our ancestors progressed slowly over hundreds of thousands of years. Eventually the population became intelligent enough to form a civilisation. Next, the forces which lead to the evolution of a higher intelligence became reversed.
Civilisation is organised to help all members of society, including the stupid, lazy and shiftless.
Therefore, these genetic defectives now survived to procreate and perpetuate their own kind, decreasing the level of intelligence in the population. Extensive research has shown that those of lower intelligence, on average, have more offspring than those of high intelligence. The unintelligent are increasing in number, while the intelligent are shrinking. Similarly, physical weaknesses are also spread. This tragic chain of events has doomed every civilisation that has ever existed.