I can't speak for the parent, but having been in that same boat, I'll answer for myself.

Why limit yourself to <language_x>?

It isn't about limiting myself to the language as it is being able to hit the ground running with it. I was mistakenly given an interview where they were looking for a Ruby developer, and had been referred to them. Only problem was that I didn't know Ruby as intimately as I do other languages. The position was fairly high paying, and one assumes an expectation of proficiency that I wouldn't be able to deliver on.

Could I have learned the language? Sure, almost certainly -- but the question is really whether or not I can learn the language faster than they can realize how inefficient I'm being and (rightly) fire me? Even if I'm upfront about it (which I was), I'm still not willing to gamble unemployment on whether or not I'll get up to speed within their expectations when I could easily justify my salary somewhere in a language I'm more familiar with.

Edit: I should also add that since that time, I have spent more time with Ruby and figured out that I don't really like it. Not going to get into a language debate or anything, it's a fine language, but it didn't fit the way I naturally think so, had I taken that job and learned quickly and all that, I'd have found myself programming in a language that I didn't really love, which could have lead to less job satisfaction as well.

Why ask children "Why can't you run?" when they just learned to walk? You have to start somewhere and it's really not easy to learn several languages at the same time, especially not your first. Eventually you'll want to run/learn more languages, but there's a time and place for everything.