To get a job at a company you have to be given the opportunity to interview, and to succeed at the interview. If you only evaluate performance at the job level,

To get a job at a company you have to be given the opportunity to interview, and to succeed at the interview. If you only evaluate performance at the job level, you don't evaluate performance of anybody who failed the interview. This is one of the fundamental challenges in hiring - you have a biased sample when trying to determine how effective the hiring process is at screening candidates.