The direct cost of manufacturing the units is only a small percentage of the total costs of the retail product. Logistics is often a bigger cost. You can create substantial economic efficiencies elsewhere in the supply chain by allowing some manufacturing waste.

Production processes that require more production and supply chain customization for each order have significantly higher costs that need to be amortized. It is cheaper to pack and ship identical boxes at the factory than to customize the contents and logistics of each box for every retailer or customer. The more variation and complexity you allow into the supply chain, the more capital infrastructure, equipment, and people you need, all of which must be amortized into the retail unit cost.

The costs of increased supply chain variability and customizability can easily exceed the cost of wasting a few units. You may have wasted hundreds of t-shirts but you also didn't have to invest the millions of dollars in systems and equipment that would have prevented that waste. These are low-margin businesses, everyone is carefully tracking and attributing these costs.

Supply chains in most industries continuously and ruthlessly optimize to squeeze out waste while trying to increase flexibility. The number of items that are produced on demand -- and therefore produce little waste -- has grown dramatically over the last couple decades. However, many goods intrinsically have long, slow supply chains which makes waste all but unavoidable.