They already are. Unless you turn it off, Alexa is always listening.
There’s a difference between “listening” and “transmitting.” We ran network trace tests of an Echo for a week and found that the device transmits data once a day if not used, and it’s a beacon check for updates. This happens even if you have the device muted. The amount of data transmitted was less than 1 kb if there were no updates.
An out-of-the-box Echo has no open ports, no management interface, and even with Bluetooth, it’s not possible to change the device without taking out its guts and reprogramming the individual EEPROMs.
“Alexa” or “Echo” or “Amazon” are keywords that the device knows to recognize, and yes, what you say is uploaded to the Amazon cloud, but other than words spoken directly after, there’s no other data uploaded to Amazon without you knowing, and they provide methods to delete recorded data.