It is a very good point especially as most of us go through some country's customs hall every time we go to work.
Our bags have been out of sight and therefore our control for up to 20 hours or so.Blind Freddy could open the usual lock and if you fly into or out of the US you have the TSA who can and do open your bags without you being present.
When asked " Did you pack your bag"...you can answer "YES"
"are you aware of the contents of your bag"....." I was when I packed it but not now as the bag has been out of my sight for nearly 20 hours"
The problem is that with most western countries that line of discussion is possible but with some other countries you might as well be bashing your head against a brick wall.