To be an effective FE, you need to understand the whole stack - not just how to install a jQuery plugin, but also CSS, Javascript the language, html, where these technologies are going (html5, ecmascript 6), what the back end stack looks like, how data gets from the database to the browser, how the browser rendering engine works, pragmatic optimizations, functional and OO concepts, version control, browser quirks, the TCP stack, and deployment. No, you don't have to specialize in these things, but you need to be familiar or willing to get familiar.
And you're right, most companies don't understand what FE engineers need or what they should be.