If it doesn't then it's kind of ◊◊◊◊◊◊◊ useless, isn't it? The device listens to everything until it's specifically addressed and then given an action to take.
Do you mean "asleep" because if you're going to shut it off that makes it rather useless.
Well, it's not. A standard "managed switch" only monitors where traffic is going from hop to hop and doesn't even know, or care, where traffic is going when it leaves that specific switch. It doesn't have to know since it's not relevant to a switch's job. A switch receives a packet, looks at it's routing information and sends it to the next hop. It might keep basic info like destination address if it's layer 3, but most of the time that's not the case. That's also not to mention that the majority of households don't have managed switches since they cost significantly more and the bulk of the population has no idea how to configure them.
If you think wireshark is "trivially easy for someone with a modicum of knowledge" then I'd suggest finding a laymen to use it and tell me how it goes. The ISO and TCP\IP stacks aren't exactly common knowledge or easy to pick up on for the standard user.
Amazon's official statement is that the Alexa is always listening but only for it's "wake word". It doesn't record anything else until after it hears the wake word, if people find that reassuring I guess. It doesn't to me, but if people are ok with it then they should use them.