The stomach digests the food due to corrosive hydrochloric acid with high pH - from two to three. But at the same time, the acid acts on the gastric mucosa, which, however, is able to recover quickly. Mucous of your stomach is completely renewed every four days.
Scientists have many versions why this is happening. Most likely: it’s because of huge asteroids that have influenced its course in the past, or because of the strong air circulation in the upper atmosphere.