Several reasons. Most notably, flour needs to be exposed to air after being milled in order to properly oxidize. While this could be done prior to being packaged, companies save time and money by packaging it in a non-airtight package and shipping it sooner.
Also, this is apocryphal, it’s a story I’ve heard, but can’t find a specific reference to. According to legend, there was a company that tried to switch to packing flour in plastic containers. But the static electricity caused by flour rubbing against plastic as it was packed let off a spark and half the factory went up in flames, because flour is flammable and light enough to get everywhere and into everything.
Flour typically has to oxidize (react with air) after being processed. This can take a while to do.
Flour is a basic commodity. It’s always needed, so it’s always bought quickly. It doesn’t have to sit on a shelf for weeks before it’s bought.
So, they just sell it in a breathable package. Let’s them get the stuff out there quickly, and cheaply
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