Amphibians have thin skin because they breathe through their skin. The problem with very thin skin is loss of hydration. To offset that drawback, amphibians choose habitats which are naturally moist and that moister accounts for their moist skins. Were they to remain out of their moist habitat long enough, their skin would lose that moister and the animal would suffer and eventually die from loss of hydration. I should add that a characteristic of amphibians is the absence of scales. Their skin is very smooth.