Bread gets hard when left out of the bag because of dehydration which draws the moisture content away from the bread.
Bread becomes hard due to dehydration of the moisture in the bread.
When the bread is exposed to the outside air the air starts drying out the bread which makes it hard because the moisture content in the bread is leaving or has left the bread.
Water of hydration - as opposed to free water - plasticizes the starch polymers in bread and if that water is removed, the bread hardens.
Surprisingly, bread will stale and harden without losing any moisture from the bag. It will seem like it has dried out, but the bag won’t lose any weight so we know the water is still there.
A great use for dried-out or stale bread is to cut it up into one-half inch cubes and toast it lightly (a counter top toaster oven works great for this, but not a regular toaster).
Put the toasted bread cubes in a sturdy plastic bag and keep them in your freezer.
Use for croutons on salads or in soups, or use them to make stuffing for chicken, turkey, duck or goose.
When they fill the bread bags, they also fill them with a nitrogen mixture which doesn’t oxidize.
That means the food stays fresher until you open the bag.
Oxygen is a reactive element and combines with all kinds of things to make things deteriorate.
Nitrogen is more stable and doesn’t react to cause the food to go stale, the metal to rust etc.
Bread goes stale due to the lack of moisture (water).
So if the bag is closed the water in the bread will not evaporate as quickly as when it is exposed.
The bag will keep the moisture trapped. I hope I explained sufficiently.