The short answer is that the insect survival strategy took a different path. The ability to reason isn’t the only survival characteristic, just *a* survival characteristic. Evolution is really the sum of a wide array of factors interacting with each other over time, and a tiny change in any one of those factors can produce profoundly different results over a long stretch of time. Some animals survived because they produced a huge number of offspring so at least some would survive, while others survived because they invested more energy in a small number of offspring.
The short answer is that the insect survival strategy took a different path.
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So reason is not an end-all be-all. Like I said, it doesn’t seem to be needed at all for a large majority of the animal biomass (of which 6 or 7 billion humans are practically a drop in the bucket) to survive, thrive, and evolve.