Here's why:
* It's a phrase, not a sentence. It lacks a verb to express an action or state of being.
* It needs context. To make sense, you'd need to add more information:
* "You and Sarah are going to the party."
* "You and Sarah should help me with this."
* "Can you help me, you and Sarah?"
It's important to understand that while "you and Sarah" is grammatically correct, it's only a part of a complete sentence.