As an artist of mediocre talent I get what you're saying, and it's flattering, but by your logic there's nothing special about a 1933 Bugatti, since almost everybody has a car.
Yes every painting is "unique," but post-impressionist masterpieces by highly renowned artists that happen to be for sale are far more "unique" than others.
so the wording of the article misleads—it means to say that a painting of such high caliber is rarely on the market and available.
but the painting itself is one of a kind.
cars are different—some cars are more rare then others. perhaps there might only be 25 known cars of a certain type of classic Ford Mustang, but there might 500 known cars of a special plymouth roadrunner production running around out there.
The Mustang is truly rare.
But the Mona Lisa is unique.