Question re: Built in Video Call

Why does the HD video call not work across carriers??? I don't understand this at all.. there's no need for Google Duo if they can just make the devices talk amongst carriers.. anybody know the specifics behind this?

I.e. I can't use the built in video call to call my gf ATT N7 from my Verizon N7

from xda-developers http://ift.tt/2cwTnVY
via IFTTT