First, the Echo is always listening for a wakeword, and just because of the inherent limitations of the device, will occasionally get wakewords wrong. By default, Android phones don't listen to wakewords when locked, and can be easily configured to require a button press before listening.

Second, the microphone on your smartphone is less powerful than the microphone on your Echo, and your smartphone isn't omnipresent in every room of your house simultaneously, including guest bedrooms.

Third, (opinion me) Amazon is a slightly less trustworthy company than Google, and a much less trustworthy company than Apple.

And finally, you can outright turn off assistants on your phone (and should). An always-on virtual assistant run by Google that always listens even when your phone is locked is also a privacy nightmare, but there's pretty much no reason to have that.

If you're paranoid about the security of carrying a microphone that could be enabled via software at any time, you can also install a custom ROM on your Android device that uninstalls Google Assistant and non-FOSS software entirely. LineageOS is really good, I highly recommend it -- it's just not feasible to install for regular, nontechnical consumers.

There were multiple instances of Echo waking up when the user didn't utter the wake word.

And smartphones are immune to misrecognizing "Hey, Siri" ?

So, in other words, Echo is always listening for that wake word?

Sort of. AFAIK it's listening with a dedicated circuit that only recognizes the specific word, not with a general-purpose listening device.

Again, how is this different from a smartphone that will wake up when you say "Hey Siri" ?