I'm not at all defending the practice, but I think it has to do with the legacy. Google is (somewhat) up front about tracking, and has always required an account. There was never a Chrome OS that didn't track you.

Windows didn't used to do this, so it feels like they're changing the rules after you've agreed and bought into the platform. If Microsoft had instead called "Windows 10" something completely different, and similarly was up front about what it was doing, I suspect there wouldn't be the same backlash. Some people would still disagree with it, and would be unhappy they were moving to replace Windows with this other thing, but it wouldn't have the same impact that acting like it's just another major version upgrade does.