It also has the peripheral advantage of ensuring that you never carry a package for someone else, and that you are leery about carrying luggage/bags packed by someone else.
In fact, I don't see why anyone would have a problem with that...it's a constant and (imo) fair assumption of mine that my friends are not trying to kill me.
Then, with images of airport holding cells and enhanced interrogation jumping to mind, I hastily explained what I meant. Interestingly, the agent was happy not to unwrap and open the present, and just inspected the rest of the contents...
Carrying eg an apple into the US is probably not going to be fun if they find it.
Also - while you can be trusted to make judgements about your own safety, the airlines have to make somewhat more conservative decisions about the safety of everyone else on the plane.
And carrying packages for someone else is just one of likely hundreds (thousands?) of factors that they are concerned about.
This doesn't guarantee airplane safety (Nothing ever will), but it reduces the number of easy attacks one can make, such that it's becoming somewhat more difficult for a passenger to bring down the plane they are flying on now. Of course, now we have to worry about the pilots (a thankfully rather rare vector)