That's not how it goes. You'd hire the perfect candidate under all measures. So that's a silly hypothetical. But maybe they have another candidate where the risk is much lower of them leaving after investing training effort, or of them getting bored or finding a job elsewhere more suited to their capabilities, then it makes sense not to hire the overqualified candidate. It's not usually this one thing, unless early on in the process and they have loads of candidates. And again, "too expensive" is also a legitimate reason not to hire someone.