Acid is not magic, despite what spy movies and the like would have you believe it doesn't just dissolve whatever it touches immediately (if it did that would be bad for your stomach for example). The acid in your stomach is for breaking down food. It will also affect other things obviously but not everything. Depending on what you ingest it can be harmful before it gets to your stomach, inside your stomach, or after it passes through your stomach. It takes time and effort (churning) for your stomach to even break down the food you eat, and even that it doesn't do completely. Take corn for example, the shell of a kernel of corn can survive the entire digestive process and pass through into your stool intact if its not chewed well enough and pre-broken down. Our stomachs aren't the friendliest environments around, but they aren't the most hostile either.