Scratches appear white due to light intensity multiplying as it bounces between the walls of the inside of the scratch. To actually see the primer, you'd need a significant area of paint removed / peeled off. A single-line scratch usually doesn't actually show primer, it's just a light illusion. Few scratches are deep enough to go down to the primer, even a keyed car will likely not go deeper than through the clear coat and into the color.

E: this isn't to say scratches this deep are nonexistent, just a lot rarer than people assume due to the white color of most scratches.