Because we as a people aren't standardized. So how can you possibly hold everyone to the same standards?
I think you can have everyone take the same tests, but even then, people test differently. So just because you offer everyone the same test, only those that do well with that kind of test will score well.
This has really bothered me because I haven't figured out how we can possibly find a solution. There's just too much subjectivity in the creation of the tests and the scoring.
I wish I had a solution, instead of expanding on the problem. But I'll keep thinking about it.