They actually already have that. The Fire TV Stick has "Alexa" in it, but you have to use the push-to-talk remote instead of having an always-on mic as in the Echo.
It would be nice not to need a remote, like with Kinect but with better mics and voice recognition
Part of the problem with that is that if the TV isn't on the right input then you won't be able to receive the response. HDMI Control to change the input exists, but with false positives it might be a bad thing still because it would interrupt whatever you're doing to listen and go "i couldn't understand the question i was asked". I'd quickly disable it if it was doing that.
I power my chromecast directly and it uses HDMI-CEC to automatically turn on the TV. It works pretty well.
Won't be long until TV's are internet connected microphones too.