In the off-the-rack production, they can cut several fabrics at the time, (sometimes even more than 100 sheets of fabrics). Then the fabrics would be brought into the sewing site, with the same specification, repeating the same work, and they can produce mass of shirts efficiently.
However, in the made-to-measure process, it doesn't work easily like that. The skilled craftsman would cut the fabric one by one according to the pattern, and customer’s size specifications. Of course, even with sewing process, according to the collar style, cuff style and each customer’s size specifications, it will take time and become an only-one shirt for each individual.