If there is only 1 job but 10 candidates, the job has to go to someone. If everyone has the same scores on an exam, what's the fair way? Flip a coin? Perhaps. What if there are intangible skills/knowledge that are important for the job? One person has a better score on the exam, another person speaks a language (or dialect) that is important for the job. Maybe 9 of 10 come from one academic background, the 10th comes from a different one...which may actually provide a different perspective and provide new insights and break group think. Maybe one comes from a culture that is more confrontational, which means they may speak out more than others.
So many factors are intangible or at least not explicit and I think that's where "merit" can become so dimensionally reduced, not realizing how multidimensional each individual is.