None of those things are "people-centric", e.g. they aren't tied to a single (other) person. People-centric UI doesn't make sense because I use my computer for interacting with many users at once simultaneously through websites, mail, etc. If I'm not doing that, I may be interacting with my personal data. And probably least commonly, I am interacting with a single person. So "person-centric" (not people-centric", despite what the title claims) design isn't taking off because I'm not interacting with one single person for the majority of my time when I use technology.