That's the point. Also the cost of verifying that your sources identified their sources correctly can be expensive.
On the other hand, most of the stuff I buy does say where it comes from. Most contries like to brag. In the US, many items are labeled by state.
How do you label things like fresh vegetables? Put them in plastic wrap with a label?
Most of the vegetables I buy in the grocery store already have some kind of a little sticker on it, I suppose to identify to the cashier which variety of vegetable I'm buying (the $3 tomato, as opposed to the $5 one).
Of course, stickers can fall off. But, I think they've engineered them so that they can only stick to something once.
Practically every vegetable now comes with a little sticker on it (very annoying.) If there's no sticker, then put it on the sign above the bin, next to the price.
It isn't rocket science; it's another payback from President Bush to the Mexicans, Chileans, etc. who increasingly supply food for the US market.