Your feet sweat. That's a fact. The moisture has to go somewhere, and some of it is absorbed by the insole. The insole is foam.
If the moisture is allowed to evaporate, no harm done. But if you keep wearing the shoes day after day, it causes trouble.
Sweat is not distilled water; it's a complex solution of salts and water. It has a non-neutral Ph, and if it's not allowed to evaporate completely, it attacks the foam. The foam breaks down, and all the cushioning in the insole becomes essentially worthless.
This is all news to me, but it makes sense. I've gone through two pairs of shoes since starting work at the store, and I'm about to start on a third. Guess I need a second pair....