I think the starch recrystallizes, which is why it gets tough and dry. The moisture is still there, it just migrates to the spaces between the starch. Heating it up dissolves the crystals. (assuming it's just stale and not dried-out, those are different things) When I warm up stale pizza or tortillas that I know is also dried out a bit, I sprinkle a little water on before reheating.