The real cost savings is they build exactly the same 4 floor plans over an over again, and very little modifications are allowed. They then have jigs to cut the pieces to the exact size needed. Instead of a carpenter with a saw measuring a 2x4, they cut everything to the exact size needed without using a tape measure.
Cost savings on the surface but when things come together they're just not there.
A lot of the actual prefabricated style solutions that do save money seem to be small changes within your traditional systems.