> I didn't say Africans and Europeans, I said black people and white people.

Same thing

> A black person from Africa and a black person from elsewhere are considered the same race because of their visual similarities, regardless of their background. That's the social construct.

And also perhaps because they actually are from the same race? Do you consider all the current inhabitants of the USA to be native americans?

> It can be inaccurate enough that it isn't useful, though.

That's true, but racial categories are really accurate, especially in this age of cheap DNA testing. As for usefulness, I'd say it's pretty useful in the medical field, for instance certain drugs work in some races but not in others due to racial differences in body chemistry.