Notice how, even in your post, you say "people talk about Google Home", but the equivalent to Google Home isn't Alexa, it's the Echo Dot.
It's possible that Google copied Baidu, which used "Ni hao baidu" [0] is the only thing I can think of.
[0]: https://www.coursera.org/learn/machine-learning-projects/lec... (see the video transcript)
However I think they missed on how personal a thing it is to talk to someone. I think they should have viewed 'OK Google' as a personality that's like Google's little brother, that will ask Google for you if that's what you need or else will for example check in your diary for you.
> Having to say "OK, Google" instead of just "Google" (or "Alexa") is a deal-breaker.
It's not good.
I'm not saying they should use "Google." I don't care what they use, as long as it's better than, "OK, Google."
“Now” as in “since the introduction of Google Home”. (On phones prior to Google Home, it only supported “OK Google”.)