Every time they ask me that question - I do a quick mental checklist, always mention that my luggage did leave my control when placed in the boot of the cab - which they always understand is an acceptable loss of control.
There are a couple alternatives that one could take to further increase security - first, presume that 100% of people are threats, and do a full security scan, and hand check/rejection, of 100% of checked luggage. Likewise, enhance the carry-on security check, and further restrict what people can carry-on. In particular, shift the carry-on check to the gate (where it should be).
I'd bet that a fair number of the people who thoughtfully answer that question give an answer that is calculated to produce the least amount of hassle for themselves, even if they know that their luggage could have potentially been tampered with. Most people probably aren't aware enough of their surroundings to give an informed answer. I'd like to know out of the number of extra checks that have been initiated due to this question, how many have yielded anything useful.
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)
People are easy to manipulate into carrying a parcel across international borders. Also, isn't it the start of gathering evidence? If you say it's your bag and you had control of it and then they find it full of contraband they can use your comments to tie the bag and contents to you?
it exists to weed out the absolute moronic and deranged. not everyone is a supervillain with an iq of 300.
plus, i guess it also exists for legal reasons. lying to the question can be used against you.
there is a lot of smart people out there, working keeping things running and safe. if something looks ridiculous from the outside, it still might be ok - you just might be lacking context to understand it.