Here's why:
* Linking verbs connect the subject of a sentence to a noun or adjective that renames or describes the subject. They don't express an action.
* Becomes and seems are both linking verbs.
* "The sky becomes dark." (Dark describes the sky)
* "He seems happy." (Happy describes him)
* Plays is an action verb. It describes an action being performed.
* "The child plays in the park." (The child is performing the action of playing)
Let me know if you have any other questions!