Because of some ancient international postal service agreement all items sent by one country's postal system is delivered by the recipient's postal system. Like every system it is only as strong (reliable/secured) as its weakest link.
That is why businesses and important personal timely items are shipped by private (UPS FedEx DHL) who are 100% accountable door to door.
Getting signature confirmation is quite costly due to the additional handling at both end whether it is postal or private and one should be prepared to pay for it if it is that important.