I believe the issue here is not that it can't be done (T-Mobile offers WiFi calling on certain phones), it's that it can't be done well. The transition between WiFi and Cellular is often far too bumpy for a low-latency, bi-directional audio stream to sustain, so the call will drop. This would result in a service that would get more complaints than praise from most phone companies' point of view. So they don't offer it.
That and probably also patents. It's always patents with this kind of shit.