Why are food bags/boxes never full to the top ?

Because, as it says on the box, the products are liable to settle.

packages are sold by weight, not volume. Says that on every package I’ve ever bought.

@evelyns_pet_zebra I know they are sold by weight a weight is a certain amount .
@Lightlyseared Yes contents will settle but since the know that they could just package it a little tighter in the bag/box .
I know that will then make a risk of bursting but that is a risk already .

I imagine it has something to do with shipping by air.

Well, they sell generic cereals in plastic bags, no box needed. Buy that instead. In Wisconsin, they sell milk in plastic bags, too. No big carton to have to dispose of. You have a choice you know.

If they packed it tighter, there would be much more breakage of the product during shipping and handling.

Maybe it wouldn’t break up more

Pre jiggling/settling the product into smaller boxes would add the the breakage and crumbling already incurred during shipping. Me, I kind of like my Cheez Its whole.

The cost of pre jiggling the product would increase the cost of producing it negating any saving you would make with having a smaller box.

i know for bags of chips they put air in there on purpose to keep the chips from getting crushed while being shipped. its probably the same thing for cereal.

It is a psychological trick.
They want you to buy their stuff so they put it into big boxes to suggest a big content.
Bigger = better is wired into our subconsciousness, they know that, and they exploit that.

@ragingloli OMG! So the companies I buy everything from are exploiting me subconsciously? That explains so much, I feel so dirty and used!

the previous sentence was playful sarcasm and should not be taken as a serious or hurtful comment.

@evelyns_pet_zebra
I don’t approve of your unwarranted sarcasm.
The brain automatically associates a bigger package with bigger content, just as it equates a brand name with quality—as mentioned in this question
An example. Whenever I go through the supermarket and see a big package of cornflakes, the first impression i have in comparison with smaller packages nearby is “the bigger one must have more content.” I don’t have control over that, as it is automatic. But as a conscious buyer I then look on the actual weight.
But the point is: Most people are not conscious buyers, and they let themselves be fooled by the package size, and then buy the bigger package, even though it might have the same or even less content then the smaller package next to it.
The number one priority of any business is money. And if they can improve sales by exploiting basic psychology and simply increasing the package size without increasing content, then that is what they will do.

@ragingloli aw, lighten up. A little innocent sarcasm never hurt anyone, and it didn’t detract from your previous comment. sheeeeeeeeeeesh, what a grouch!