It's mass-production sewing, just with non-uniform stock. The customized pattern cutting provides the customized fit. Instead of boxes full of identically-cut pieces to be assembled, there's a packet with someone's customized pieces. Which is why the US getting out of mass-production apparel manufacturing was a problem.

Perhaps this was a niche in which US apparel manufacturing might have remained viable, with customization and shipping proximity supporting the extra US costs. It seems regrettable that the possibility wasn't better explored.