again, this is because the idea the complexity can develop from chaos was very hard to understand before Darwin. now we understand how adaptation works. now we see that if, for example the world became much hotter, future humans would develop more efficient Cooling systems, more capillaries closer to the skin, larger ears to radiate heat maybe, whatever. animals adapt to their environment, they are not created and placed in the perfect environment. a camouflaged snake developed that camo over time, he wasn't just put there by god because god noticed he looks like leaves and it would be smart to make him hide by looking like leaves. the snakes that looked a bit like leaves lived longer and had more snakes, and of those the ones that looked like leaves had more, etc. we only recently started understanding natural selection, and how complexity evolves and adaptation works. so primitive cultures had no understanding of anything, they just made up the answers. they also thought evil spirits caused the flu or whatever. we don't need to think that anymore. so after Darwin one of the most vexing problems is answered, and you have less need to make up a god to explain everything.