Comment by tsimionescu

Even in this idea of "could in principle", the IP address randomly changing as you roam is not an arbitrary technical limitation, it is a fundamental property of how IP networks operate that proved to be too hard to fix for IPv6. The LTE network does a lot of work to try to preserve your IP as seen by the servers you connect to, to preserve your active connections, but this could only be done as a layer on top of IP.

The reason why this is fundamentally quite hard is to do with routing optimization. Routing tables have to reflect physical connectivity - which means packets destined for F2::67 mustn't be routed to LA if that is the IP of a phone that is now resident in SF. However, changing routing tables on all devices along the way as a phone roams would not be an easy feat.