oh I am sure most are delivered somewhere, just sometimes not to the right address. It probably depends on the post office and delivery person, just like other professions some will be better than others. I know I had a hard time finding good workers when I was in the workforce, in today's job market it is probably worse.
In the last 20 years I had some steel bar from McMaster delivered to neighbor on next street over, neighbor found it and returned it, same with a package from Midway. I have done the same for a couple of neighbors on at least two occasions. Over half the streets in my subdivision end in "dale" so it is understandable.
When I returned my Labradar to Brownells it made it to the town where Brownells is located and went into a black hole. Three days after it hit the local post office and not to Brownells receiving desk Brownells declared it formally lost and returned my money, no idea if it ever made it's way out of the black hole. Got to love Brownells
Overall percentage is probably less than a half a percent but all it takes is once to ruin your day