I think part of the problem is that there is no infrastructure for something like this on iOS and Android.

With webOS, there was a single messaging app that integrated SMS, Skype, Facebook and Google. The latter part was implemented using libpurple, so it was relatively easy to extend this to all messaging that libpurple supports.

I really liked that approach (marketing name was "Synergy", unified messaging was only a small part of this), but it's quite obvious that neither Apple nor Google have any interest in adopting this unified approach for anything but their respective own messaging of the day.