Actually it does. It does not generate any power and is dependent on some outside power before it's resistance change can do anything.
Photo diodes can produce current when subjected to light, LED's ditto.
Is this all news to you? are you just being obstreperous?
I saw one optocoupler datasheet that uses a series string of integrated photo diodes to produce the voltage necessary to bias a mosfet on... here it is:
Now, do you want to rethink the idea that they need power?