No, I don't think they're invented all the time but the languages do evolve and change over time. If they stope evolving, they become dead languages.
Sometimes a language will give birth to a new language; Old French Sign Language is said to be the parent language of ASL. Latin is said to be the parent language of the Romance language such as French, Italiatin and Spanish.
On the other hand phrases can be invented much more easily than languages. Pwned comes to mind as a recent example. I believe it's also a lolcat? I've tried to look it up but i was unable to find any info on lolcats.
In order to invent a language, you'd have to think about the structure and the syntax of the language and decide what rules should apply to it. That's a lot more work then just sticking with the orginal language. Also, it takes time to learn a new language, hence why it's much harder to invent a new language than to stick with your mother language.
