[cyphar]

The most likely explanation to me is (4) -- but it's because flour (et al) producers are struggling to get their flour into the consumer supply chain. Shipping goods to businesses is quite different to shipping it to consumers -- businesses order in bulk and (to be frank) don't care as much about trivial things like packaging or minor product differentiation. Compare the giant packages of cheese and cured meat at your deli counter to the small individually wrapped packages you get elsewhere.

Costco is the exception here (their entire business model is to buy things wholesale, from the business supply chain). Most grocery stores can't stock 12.5kg bags of flour because they don't have enough space on their shelves, not to mention that in their view most consumers would be put off by a lack of choice (even if it means that products are in stock).