Buyers are usually upset with the limited choices they can have. For instance, they want to order 3 tailors of shirts with 3 patterns designs, 4 color combinations each patterns designs, 1,000 pieces for color combinations. They want to order more models with small quantities, so their e-shop or paper categories will has a large range of model collection. They will be told by the factory, that this isn't going to happen. Why? The reasons are sort of a repetition of those for problems mentioned above.
A. The sellers have to source materials and parts from their suppliers. Their suppliers also have MOQ requirement for each part and material. They can't purchase a trace amount of them just for your little order.
B. It will make them countless troubles in production, and the scheduling and arranging for production, management and communication can all be a mess, if they try to make so many small productions at the same time.