When God was creating the world, she had all the ingredients necessary. Since the plan was to make an impression, she got going and started building everything structurally. Top-down approach. Sky first, dust later. It took several centuries to complete the task. With no one to assist or blame, she had to do all the whole thing. And finally, she came upon our species. At that point, she had depleted all her resources and was left with only skin and poison at her disposal. She thought to herself, "This peculiar creation of mine shouldn't have poison in it. It should be as harmless as possible." So as to divert the share of poison, she haphazardly created two snakes out of thin air and deposited all the poison in them. Later, she turned her attention towards humans. "My finest creation. Woohoooo!" she exclaimed, not realizing she had accidentally granted the gift of ceaseless skin to the crawling reptiles while the two-legged beings would wrinkle while consistently poisoning her world.