There is a distinct advantage to serial numbering parts
You don't mix up bolts and lose headspace, the wear patterns of parts are matched, German weapons may have required a slight amount of hand work or fitting and this isn't lost with a part swap. Lugers are that way, 98Ks etc. US level of manufacture was beyond good. The same situation was around with watch parts, American watches needed no hand fitting as all the parts were perfect and repeatable. It would also be very expensive to mark each part like the Germans. Easy to see why they did it.
Thank you for all the great info
Thank you to everyone responding to this thread. You've given me a lot of good info to digest.