1) [Leverage]. The interviewer can negotiate down salary expectations based on the performance of the interview. You can ramp up requirements until the candidate breaks. Then, you get the upper hand in the negotiation process.
2) [Responsibility]. Nobody wants to take responsibility for the bad decision. In the formalized interview you can blame the metric. If the metric is external (Google/Facebook/Amazon do it), then there is nobody to assign the blame, so, no responsibility gets redistributed.
3) [Effort]. Treating people as they are different is hard. Reading their CV or learning more about their projects requires time effort and expertise in the field. This is expensive. Much easier is assessing people through a generic process.
4) [Loyality]. Someone, who spends their free time for a year preparing for the interview is bound to be more loyal than somebody who prepares only for a week.
5) [Initiation]. People who go through a difficult initiation process tend to value their group more. That builds tightly knit teams.
Overall, I think that the process is difficult, somewhat irrational and inhumane. But I have no idea how to really fix it.