I'm not saying it only happens on exports, nor am I arguing it happens with only olive oil. I'm responding directly to your claim that "high quality dop/docg olive oil is readily available all over the world."
It may be available, but readily available seems like a bit of a stretch to me. It can be deceptively difficult to obtain.
> It's reliably reported that 80% of the Italian olive oil on the market is fraudulent.[0]
[0] https://www.forbes.com/sites/ceciliarodriguez/2016/02/10/the...