A successful organism would not have evolved to highly express cortisol to weed itself out in the case that it has a stressful life. It makes no sense for an organism to evolve a trait that makes it at best equally likely and at worst less likely to reproduce.

The reason we release excess cortisol in modern life is because our bodies did not evolve for constantly present stressors, they evolved to be stressed in a situation, run away from the tiger, and then you’re good.