Google can do that because the phone with that feature has an extra low energy CPU. Have you seen this feature on a s5 or HTC?? No, because they don't have this CPU. That's why Apple is doing it this way. Without plugging in, the battery would be 0 before you can say cortana.
And if all your friends own an iPhone? 1 person says: „Hey Siri” and all the phones respond.I just read that iOS8 will allow Siri to wake the phone from sleep mode when you say "Hey Siri."