I will guess that it's because there are a lot of candidates that will do fine in the more "human" parts of the interview, but cannot solve FizzBuzz. It's just

I will guess that it's because there are a lot of candidates that will do fine in the more "human" parts of the interview, but cannot solve FizzBuzz. It's just more efficient to screen those out quickly, and save the human interviews for those who have already established a baseline technical capability.