Because all evidence[0] I've found has shown that work-sample tests and repeatable, structured indicators are the best indicator of job performance. I understand that a lot of devs think they can get a feel for a candidate by just having a good ol' conversation with them, but the body of study on the subject says that that's just wrong. And so I have people do a task that's as close to the actual job as humanly possible and evaluate them on that.
https://www.researchgate.net/publication/283803351_The_Valid...