Here's why:
* Articulation: The act of joining or connecting, as in "The articulation of the bones in the knee."
* Articular: Relating to a joint, as in "Articular cartilage."
* Articulate: To express clearly, which comes from the idea of connecting thoughts and ideas.
While "joint" itself isn't a root word, it's derived from the Old French "joint," which itself comes from the Latin "junctus," meaning "joined."