Depends where you are I think a lot of Scandinavians have it in their genetics but also I think it is rare to see whites with a shall we say a "Pure" colour they are more likely to have shades of it. Why I am not that sure, maybe shades or a mix of colours is more appealing to a white persons eyes.
If you are too pale, you will look like a corpse once you dye black. If you are to dark, you will look like a gypsy (no offense to gypsies). So black hair looks best on people who have perfect pale skin, but with bunch of make up so that they don't look dead.
Jet black rare isn't as common as you think. I have what I consider black hair, but it's got a good amount of brown/red glow. I think East Asians are more likely to have jet black hair. If you're wondering why more pale girls don't dye it jet black, I would say it's not an easy look to pull off. It looks very intense just like beach blonde. But bleach blonde is associated with sunkissed glow pretty girls whereas jet black is associated with goth/emo so maybe cultural views play a role.
A lot of white people don't dye their hair jet black because it makes them look ghostly pale, which many believe is not a good look. It is rare for them to have naturally jet black hair because I guess that gene doesn't run in many white people's blood. My grandfather had jet black hair and blue eyes, which is said to be a rare combination, but six out of his seven kids turned out blonde.
I have a friend who is really pale and has natural black hair and it didn't look good on her (she looked like a ghost), then she dyed it blonde and now she rocks it. Black hair is a better suit for people with darker skin tones.