Another way to look at this change is that humans gradually figured out that they should not treat duplicate parts like one-off parts. You do not need a master craftsman to cut each duplicate part as if it were unique; if you can set up a repeatable sequence of restricted movements, you can simply repeat the same sequence with each part. And if you can preserve the setting of each tool, so that a tool change does not destroy the setting, but rather lets it be indexed back into position whenever needed, then you have saved vast amounts of time and effort.