Well, it makes a little less sense on phones, since unless you switch the SIM card, you are still treated as the other person as far as the phone network is concerned. It would be a little odd to be logged in as one person, but the phone network treat you as the other. And since there are features like importing and exporting contacts from the SIM card, those would be more complicated in multi-user scenarios.

I think there's a certain sense that a phone is a much more "personal" device than a tablet. While it would probably be possible to add multi-user support, it would be more difficult, and be less useful, than on a tablet.