Controversial but I agree. The #1 thing some firms look for when hiring trainees is if they like you, not necessarily if you are the smartest. And its not always the Partners that dont like you - you can get great feedback from your partner/associate interviews but GR may dislike you for whatever reason. This was exactly the case for me when I applied as a trainee for the firm I paralegal at. My department was shocked when I didnt get it, especially since my interview feedback was really good. After speaking internally to people from other departments, the consensus was that GR just didnt like like me, even though my performance as a paralegal had been great thus far and I did very well in my interviews - my point is, it’s not always your performance that they dont like. It might be various factors outside of your control, some may be unconscious biases GR or an interviewer hold against you. Or you might just not fit the trainee profile for that specific cohort.
And to reflect the point above - I agree, they dont always hire the smartest people. Colleagues have confirmed this as well. They hire the people that are easiest to work with, which definitely doesnt have to mean the smartest. Which is why so much emphasis is put on group tasks in some firms.