People are terrible at forecasting actual requirements, much less fulfilling those of others. Never mind the incredible stockpiles of munitions, fuel, food, water, spare parts, and replacements required to sustain a real offensive.

Huge supply depots are efficient. They are also incredibly vulnerable targets, whose loss can be crippling. A plethora of small depots are inefficient, and each is vulnerable to a modest attack. But an attacker is unlikely to destroy them all, and the loss of a few is relatively insignificant.