The stomach replaces its lining every few days

The surface cells of the stomach turn over in a few days. The acid and the enzymes would digest the organ if the lining were a permanent structure.

Gastric juice is hydrochloric acid strong enough to denature protein, plus pepsin, which cuts that protein up. The stomach is made of protein. What keeps the two apart is a layer of mucus and bicarbonate, and a population of surface cells that live only a few days before being shed and replaced from stem cells in the glands. The deeper cells of those glands turn over more slowly, on a scale of weeks to months. The face of the lining is the part on a schedule of days.

When the replacement or the mucus fails, the acid does what the chemistry says it should. An ulcer is the experiment. The organ is not a ceramic crock that happens to contain acid. It is a living surface staying a day or two ahead of its own contents, and the entry on the acid is the other half of the same fact.