Labor onshore is expensive. If you only need a few it doesn't matter but once you have offshore labor they are flexible enough to handle changes. Onshore manufacturing implies a large amount of automation, but that automation limits the changes you can make and still fit into the automation. If the change is something that is CNC, then you can make the change easily. However if the change needs a whole new jig, making the jig is more expensive than having a skill machinist make the part by hand with a file - those skilled machinists are cheaper offshore. You pay off the jig over thousands (or millions) of product.
Once you have production offshore it is even harder. They may have made a few jigs (not as many as labor is cheap enough to not need them), but and odds are even in the worst case they can re-use all but one of those jigs. Thus once you are offshore it is better to let offshore do the prototype work.
Once you have production offshore it is even harder. They may have made a few jigs (not as many as labor is cheap enough to not need them), but and odds are even in the worst case they can re-use all but one of those jigs. Thus once you are offshore it is better to let offshore do the prototype work.
It gets far more complex than that.