Add in the Thai supermarket monopoly factor which, unlike in the West, removes competition and we stand no chance!
The high pricing is clear when I shop in the few small independent Thai owned supermarkets where I stay, where most items are not just 1 or 2thb cheaper but 5/10/20 thb cheaper than in the large national chain supermarkets. With the power of bulk buying the opposite should be true as it is in the West.