Why is it unhealthy to eat acidic foods?

Everyone says its unhealthy, and can cause stomach ulcers, etc. But what I don't is why this is exactly, if gastric acid alone is already pretty acidic (pH 1-2). Adding an acid of pH 2 (ex. lemon juice) to another acid with pH 2 (HCl) doesn't make it more acidic, the pH stays the same. so why would it matter if you ate a lot of acid? wouldn't it be a problem rather, if you ate very base foods? is it a problem for other parts of the digestive system?