You haven't been paying attention. I've been working on products which exchange HL7 messages between organizations for 20 years. It's very common now. Some configuration work is usually needed on each interface but that's mostly a solved problem.
"some configuration work is usually needed on each interface" is the key phrase in your response. Just because many organizations have poured cash into yours and your competitors' solutions when the economic stars align does not mean that message passing between organizations is a solved problem.
That said, in my opinion the two organizations looking to exchange messages would need to have a pretty good relationship and a real business need in order to pursue this strategy. They may also have to compromise on dictionary values, etc., in order to have comprehensible data on both sides. For organizations with even a modest amount of rivalry, I can't see them spending this kind of time, effort and money.
My understanding was that "meaningful use" would make interop much easier, perhaps on a higher level than HL7 messages. In my experience this has not been the case.