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Well, Android is built on Linux, and iOS is built on top of a BSD variant, so it's likely extremely easy to implement at least users...they probably just haven't pushed for it, because there's no real added benefit.

If they did it where you could sign in with your phone or the remote, now that would take some real engineering, and they'd be able to advertise how cool that is. Like have a kid sit down to the TV, unlock his account, and it's instantly all his favorite shows, then he gets called away, and the dad sits down and with a gesture, it's switched to his account, and he puts on football or something, etc.