Here's why:
* Transitive verbs require a direct object. This means the action of the verb is done *to* something or someone.
* Example: "I listen to music." Here, "music" is the direct object of the verb "listen."
While you can sometimes use "listen" without a direct object (like "Listen! There's a knock at the door."), in these cases, the object is implied. You're still listening *to* something (the knock).