Epic Bridges sends/receives HL7 messages to just about any software with an interface. I'm not sure what you're talking about.
In addition to that, they are well underway to supporting HL7 FHIR (the new version of HL7).
When I can do bidirectional programmatic interface to Epic let me know. I won't hold my breath.
I wasn't on the team that implemented this at a large healthcare provider, but i remember that team bitched about it a lot. its also mentioned as a compliant on EPICs wikipedia article.
That's because healthcare interfaces are terribly difficult to work with, because they're the worst of two worlds - mountains of bureaucracy mixed with different tech stacks from different eras.
You don't have to search too hard to read horror stories about interfacing with Epic. In practice EMR vendors are financially disincentivized from interfacing with anyone.
To your point, why, specifically, is it so difficult to interface with these systems? Could it be by design?
Right, because you're searching for stories about interfacing with Epic and they have 50% of the patient marketshare. That's how appeals to popularity work.
You still didn't elaborate on why you said Epic has no HL7 integration.