But because different systems have evolved differently over time, the schemas are different, and so transfers remain painful.
It's not even that. One thing I learned while working on a project that wanted to pull EMR data was that different hospitals could have their own schemas. One division in the hospital found that the standardized codes for what they were doing weren't robust enough and invented their very own coding system which was used in that single division of that single hospital and nowhere else.
Good luck translating that to any other coding system anywhere else.
I'm not sure I can even blame them for creating their own coding system. They're doctors who found that the tools available didn't meet their needs and found a solution. Down the line it makes data transfer more difficult, but is that something doctors should really be concerned about when they're trying to accurately record medical information about their patients?