Because there is no common definition for blackness.
We code a lot into “black” in American English. Those frameworks don’t map to other cultures. A Black American, black Brazilian, Lagos native, Tigrayan, Darfurian and South Indian may have identical skin tones; that doesn’t make their cultures comparable or imply they’d each identify as black.
But the rest of the world doesn't share American culture.
I'm British as far back as the family tree goes, and I've been surprised at least once by a person revealing they faced racial prejudice for "being black", and my surprise was because their skin colour was so close to my own, mine being approximately that of Colin Powell in this particular photo (given my current display brightness settings etc.): https://upload.wikimedia.org/wikipedia/commons/2/22/Colin_Po...