Reminds me of a science fair project I read about some decades ago... Kid got the loan of some recording accellarometers and shipped them through the US Postal Service using various classes of shipping. Working from memory, the one sent First Class recored drops up to 3 feet. Second Class, up to 4 feet. The lowest class used gave no data because it came back smashed flat...and that takes some doing as the devices are pretty rugged.
It could be that so much is in the middle of the bags which get rough treatment that most is generally well protected and only an small amount will ever get damaged.