It's because you REALLY don't understand costs. Stocking has other benefits other than saving on upkeep costs. The people that win Alliance are the ones that are able to destock and sell down their stockpiles at the right time, while everyone else loses money on the food they bought. Reselling brings a hefty amount of profits as well.

Going back to costs, you probably aren't aware that upkeep costs aren't linear. If you have 1m turrets in a country with 10m NW, vs the same 1m turrets in a country at 20m NW, the 1m turrets cost more money every turn to maintain on the latter country. Go ahead, try stocking at 50m NW, vs someone stocking at 10m NW on Alliance. Not only do you have 10x more units to maintain, the upkeep is probably 15x more than the smaller country. Your income would probably be less than half of the former country after deducting upkeep.

Also, I don't know if you are in Tourny Game A. Because if you aren't in Game A, you are playing against bad players. (Every reset, the top half of Game B/C gets promoted into Game A, the bottom half of A gets demoted into B/C).