It's because culture-fit is the single most important aspect of any hire
Followed by teachability/ability to learn
Sure, technical ability (in whatever role you're trying to fill/enter) is important, but if you clash with the team(s) you'll be working with, it doesn't matter how "good" you are - as I wrote several years ago[0], "all-star teams" are horrible: what matters is that as a team you can [very close to] "the best".
Having a series of individual interviews can help ensure that no one's inherent bias (pro or con) unduly affects the hiring process
Followed by teachability/ability to learn
Sure, technical ability (in whatever role you're trying to fill/enter) is important, but if you clash with the team(s) you'll be working with, it doesn't matter how "good" you are - as I wrote several years ago[0], "all-star teams" are horrible: what matters is that as a team you can [very close to] "the best".
Having a series of individual interviews can help ensure that no one's inherent bias (pro or con) unduly affects the hiring process
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[0] https://antipaucity.com/2014/06/25/who-wants-an-all-star-tea...