Shouldn’t an aptitude test measure the same thing that the person would have to do in the job?

Sometimes YES. If the candidate must know specific things prior to being hired, the test should present questions about actual job content. For example, a bus driver should be a competent driver before being hired into the job.

Usually NO. In most jobs, people are given the opportunity to learn most aspects of the job during on-the-job training. In this case, it would be unfair (and possibly illegal) for a job candidate to be expected to know exactly the same things as a current employee. Therefore, aptitude tests for selection purposes are good predictors of whether the person would be quick to learn new job information, capable of using that knowledge to reason through problems in the job, and efficient in managing that type of information.