We mostly know what makes it taste different. The problem is, most of it is expensive and time consuming. People would like to take cheaper approaches and market as the same thing.
The terroir aspect is often overblown but does contribute. Far more often the issues is something like "milk is milk, right?" which just isn't true.
That seems like a clearer description of the problem. The issue isn’t so much where it comes from as how it’s made. Someone in the original place of origin could cut corners too.
The EU system is pretty weak, but it's the result of trade offs, like everything else.
You could be more strict, as France and Italy do for wines for example: italian wine can come with either an IGT label ("made in the right place") or a DOC one ("made in the right place in the right way") or a DOCG one ("made in the right place, the right way, and we actually checked the specific production").
The alternative is to have a local consortium certify the producers/products: parmigiano reggiano can only be labeled such if the local Consorzio checked it (all wheels are checked).
The PDO system ensures that there is no abuse cross-border (no polish "parmesan"), while the Consorzio guarantees the quality (no corners cut).
Worth saying that you can have great products with no label and made wherever, but the system exists to avoid someone capitalizing on the work of others.
The PDO approach impairs competition by making it impossible to legally refer to food with a name that people understand.
In the US we see similar complaints arise from time to time when various agencies declare that you may not name your product "oat milk" because milk is defined as cow milk that is fortified with vitamins A and D (skim milk that is not fortified, for instance, gets to be named "imitation skim milk" or "imitation milk product"). Similar complaints abound around labeling meat substitutes as "burgers" (Tofurky had to sue the state of Louisiana over this one). French wine regulators have even sent sternly worded letters to the maker of "Cat Wine," an artificially colored liquid catnip product sold in novelty bottles (they objected to "purrsecco", I believe it was?)
There is something real all these product-identity labels are protecting — it's just that over 80% what they're protecting is some producer's grasp on the market; actual customer benefit is a distant second.
> The PDO approach impairs competition by making it impossible to legally refer to food with a name that people understand.
No, because the name of the food implies a tradition and a terroir, which copies wouldn't have. If you say you're selling "parmesan" while actually doing some chemical thing that tastes like it and has the same texture, people will imagine wheel of cheese, Italia, which in that case will be wrong.
> Someone in the original place of origin could cut corners too.
This is often part of the domain control also though, and producers are audited to avoid it. But there is a real range and this also depends on the product, heavily.
The terroir aspect is often overblown but does contribute. Far more often the issues is something like "milk is milk, right?" which just isn't true.