I don't think this is exactly the issue the interviews are trying to prevent... As mentioned by a commentator below, once somebody is hired it would be extremel

I don't think this is exactly the issue the interviews are trying to prevent... As mentioned by a commentator below, once somebody is hired it would be extremely hard to get them out, especially for big corps. "Project failure" is an extreme case but the loss can still be huge. Therefore the employer just wants to be rather safe than sorry with a set of standardized tests it seems. Also, to be able to thoroughly study for and prepare for something on one's own, this on itself probably illustrates a lot about this person's qualities already. Of course it would be theoretically nice to spend all this time and energy on something that is unique to every person, but that is not totally realistic and would be hard to evaluate. One can still do that with their side projects though.