When people talk about hiring web developers, it always seems like there is such a strong emphasis on them knowing one and only one language. Like Kevin Rose s

When people talk about hiring web developers, it always seems like there is such a strong emphasis on them knowing one and only one language. Like Kevin Rose said: "you end up with lots of PHP developers, but at some point, PHP isn’t a problem anymore and you are stuck with all of those developers." Was PHP the only language these people knew? And they were incapable of learning anything else? I don't get it. I don't call myself a "C++ developer" even though I use C++ every day at work. I know several languages, and I am confident that if I wanted to start learning a new language tomorrow, I could get up to speed on it and its relevant libraries fairly quickly. Were these so-called "PHP developers" completely incapable of learning Python or Ruby or some other language? I don't understand how you can be a software developer and only ever use one language in your life. But for some reason this seems to be common when I read articles about web development.