I think apple wants to make iPhone as the centre of their communication / services hub. Everything else seems to be hanging off of the iPhone in the whole ecosystem.
Honestly, not a bad idea. I would love to be able to pick up my iPad and see health data as well as my Activity app and see what I’ve done for the day.
I could see the Health app coming to iPad, and even macOS, but the Watch app is already a separate app. Bringing the Health app wouldn't automatically mean that device would pair with Apple Watch.