You have a perfectly crunchy bag of chips. You open it, you leave it for a few days, and then for some reason its soft. What makes it soft?
If its air, then how come the air inside the bag doesnt make the chips soft? Bags of chips arent vacuum sealed like cans and jars, so what is it about the outside air thats so different from the bag air?
As the bags are sealed, they’re “flushed” with dry nitrogen rather than just leaving plain old air with its humidity inside the bag. They also pressurize the bag just slightly more than atmospheric pressure to provide a cushion so the chips don’t get as badly smashed up in transit.
Also, there’s a lot more air outside the bag than inside.
And more humidity.
StG
A visit to a 325[sup]o[/sup]F oven for 10 minutes would re-crisp them nicely.
And the salt on the chips will absorb the humidity from the air.
It contains moisture.
When the chips suck up the moisture, the outside air circulates and brings in fresh moist air. That doesn’t happen with sealed chips, it reaches equilibrium and no more moisture is absorbed.
I think they get moist from absorbing moisture in the air (which we refer to as ‘humidity’). Ha ha.
But seriously…
If you have a soft pretzel, and it gets stale, it’s because it loses moisture. For chips? They take it on.
Dry out yer chips, as suggested.
For soft pretzels, run them under the faucet and re-bake them. If you perfect both techniques, you might find they both taste better the second time around. (My WAG is that at the cell level, the second round of heating/cooking breaks down the cells more, freeing up more flavorful goodness. That’s a technical term. Look it up! 
… )
Or maybe this second cooking help just because you are getting super-fresh products: Chips that have minimal moisture (freshly dried/baked) and maximum moisture (soft pretzels freshly moistened).
This is exactly what I was going to say. Crisp (chip) packets weren’t always filled with nitrogen, and they still kept the contents fresh. The small amount of air inside the packet only contains a small amount of moisture - once the potato crisps have absorbed it, there’s no more moisture coming in to make them chewy.
Once the packet is open, new moist air can constantly flow in, resulting in a disappointing snacking experience.
Open the bag and do nothing, … and let the chips fall where they may.
Would that wash away some of the flavor though?