Chef is correct on the shaping. If you look closely at both the shoulders and legs, there is a reasonable amount of shaping in them.
There are some extremely crude versions of the shoulders that have plates that are almost flat, but almost all of them have far more shape.
The ones I have played with (and the single shoulder from one which I own) show what you have to do to make one of these really work. All of the plates taper from back to front (they are not just 'bands'. The upper plates are dished - especially the second one. the whole unit tapers from the top where it has to match the gorget down to the bottom plate which needs to fit roughly to your bicep.
Once you have done all that, the plates are attached by sliding rivets in the back (as mentioned), and leathers in the center and along the front. You can often find pieces like this where the front is attached by solid rivets - those were added in the 19th or 20th centuries to make the pieces display nicely when the leather broke down. I have some pieces like this, someday I will strip them apart and put leathers back in. All I need is some good buff leather and the time....
Just in case you are tempted to try a set of these shoulders made from strips... We did it once. It looked great - just like the pictures. It just didn't move

Once you work out the shaping on the plates, you really can make the parts for these quickly. They don't require pointing to hold them on. They don't really have to fit very well. All reasons that they were made in large quantities for munitions armour.