Maybe a misunderstanding of what's meant by cargo. Most people's idea is probably the containers or palletized stuff loaded by forklifts and so on. You won't see much of this on passenger flights except at large airports with wide-body airplanes. But there are certainly unaccompanied items going into the holds of 737s, A320s and the like everywhere, indistinguishable from baggage to most observers. And the mail has been mentioned several times.