That's the point, most of the day you're not plugged in. I know it works when plugged in, but the Android phones that I named and later ones in their series) work without being plugged in.

And yes, you use Siri hands free so rarely because it doesn't work (as you acknowledged). That's not the case with Google Assistant which works really great for most people - and I use it extensively (almost all day) - both with my phone and Google Home.