I can't get why stare should be used in the passive voice after essere. Look at these examples:

La macchina è stata riparata da Max;
La carrozzeria è stata rifatta;
Due camicie sono state lavate;

My book says:"The passive voice in Italian consists of a form of essere plus the past participle, which agrees in gender and number with the subject of the sentence". It doesn't explain why stare is necessary.