Ironically, there are two nationwide networks for sharing patient data - CareEverywhere and Carequality. They even have a bridge between the two, so in effect it's one large network.
But that's all predicated on the provider's EHR being able to talk to it, or at least talk to an interface engine that can be configured to talk to it. And money. It costs money.
Every modern EHR supports those nationwide networks. Many provider organizations have been too lazy to set it up or train their users.
Oh, you say that, but...
"Standards are anything but". I used to work for a company that wrote healthcare software, from EHRs to claims benefit management.
And our software was riddled with little transformers, because vendor A's implementation of HL7 behaved differently to vendor B's, and C's.
That's a different problem. The national networks have standardized APIs. Obviously the content returned will have some variations depending on the original source.