Why is the assembly of the door cabinet doors is done in advance?

Cabinets of coupe have now become popular among buyers. The volume of their sales is constantly growing, giving all manufacturers huge profits, and the owners are joy and pleasure. Moreover, now everyone can order furniture so that it becomes an addition to the interior and in no case change the surrounding environment. In addition, for simplicity and accuracy, the assembly takes place at home from the buyer and takes a minimum of time. Nevertheless, the doors of the cabinets of the coupe are still gathering at the manufacturer, which is required by technology. What are the reasons for this?