I think you get it from a multitude of reasons, including allowing interviewers to come up with bad faith assessments on culture fit and other un-quantifiable employment parameters that amount to nothing more than, "I liked or disliked the candidate." I'd even extend that logic to a larger cause, allowing coworkers (i.e. same level individual contributors) to interview candidates, as I don't think coworkers necessarily have the proper skills or incentives to neutrally evaluate future coworkers, especially not beyond, "I like this person because I can relate to them because we look the same, have the same interests, etc." This isn't strictly ageism either, it can affect people by class or race or other things that set them apart.