Individual takes test -> type is based on result. We can assume that everyone who is the same MBTI type would answer the questions on the test in a similar way or in fact DID answer them in a similar way. So there is a real difference between groups.
Race is based on being born into a group. Can you assume that all members of the group think the same way? Not really, it's entirely too broad, though they have similar cultural experiences.
Also, MBTI groups are personality categorizations, so personality assumptions make sense (though MBTI is rather flawed in some ways). Race groups are based on...race. Personality is an independent variable.