That's because, once you get past the low-end rubbish wines, there isn't actually much difference. Look up Rudy Kurniawan, the wine forger, who sold tens of millions of dollars of fake super-high-grade wines, certified by wine experts as genuine, that were actually just random stuff he'd bought in bulk and filled into bottles with fake labels. He only got caught when he sold more Chateau something-or-other than actually existed. Apparently the only way to tell a $500,000 bottle of wine from a bottle of $5 wine is by analysing the label.

After he was released from jail he went back to counterfeiting wines as a party trick, but this time letting people know they were fake. Most people can't tell which is the real one and which is the fake.