Even if the problem stems from incompetent developers I think you grossly underestimate the scale by multiple orders of magnitude.

This reads more like a "Tell HN" rather than "Ask HN" because you pose a rhetorical question. Lack of incentive, competition among providers, and no regulatory framework can explain the "why." If you think of health care providers as companies at least partly in the data collection and brokering business it makes more sense why they treat health records as their proprietary property.