Test scores don't identify successful people, but our system is certainly set up to make it easier on those who do well on tests. I first noticed this as a freshman in college when (because of AP exams) I moved right into deep computer science while my classmates had to spend time on general education requirements. When I interviewed for my first internship, I knew algorithms and data structures while everyone else only knew basic sorting. Once I got to my internship, I was getting paid $3 more an hour than my colleagues because of the additional course credit I received from AP tests. What's hilarious is that someone who went to MIT with the same AP scores would actually have gotten paid less than me because MIT accepts almost no credit...

You can see how the above could snowball into a huge advantage upon graduation from college. Any correlation you notice between high tests scores and successful people is most certainly a result of this property of our system and not from tests themselves. The tests could be targeted towards an arbitrary skill set and those who did well would still be more successful because the system values those who do well.