(Context: I'm a hiring manager; my team builds big distributed software systems. Our choice of language is Scala, but the team chose to use Scala before anyone on it actually knew Scala, and we don't have strong preference for Scala for software developers we hire -- in fact, we don't look for specific language knowledge at all, but rather strong fundamentals (OOP, distributed systems, etc)).
Assuming you're not looking at a company that's gaming the system (others have talked about the whole "I want to hire/promote someone specifically but I have to post a position so I'll post a position only my preferred candidate will satisfy" scenario), the other problem -- and I think this is a bigger issue -- is that most people are just bad at ferreting out talent as part of the interview process, and therefore opt for asking about very specific skills, because testing for very specific skills is actually much easier than testing for talent, for experience, for understanding of the system. Add to that, of course, that if/when your HR group is responsible for job descriptions, quite often they can't conceive of a more flexible, open-ended description because they can't effectively measure for that when filtering resumes.
The unfortunate thing, of course, is that in the end the specific knowledge is probably not even what you're looking for -- certainly, it's not what we're looking for because what we want is the ability to solve very hard, complex, problems -- and these are the sorts of problems that are also hard to ask about in an interview, because any problem you can make significant headway on in 45 minutes is simpler than what we deal with. This really comes down to the fact that interviews are a test, a simulation of a reality (the person actually working with you), and people sometimes opt to build the interview (and the pre-interview process, like the job description) in a way that makes it easier to conduct that simulation, rather than in a way that makes it more representative of the actual thing for which you're testing. It's that "looking for your keys under the streetlamp because that's where the light is, even though you lost your keys in the dark alley" problem.