That's exactly what we're doing with our Fasten Health PHR[0] & with our Fasten Connect[1] product. The biggest issue is that there are 250,000 registered health systems in the US. That's a massive long-tail to support and integrate with (we only support 30,000 at the moment). It requires a ton of time and effort, and at the end the Patient experience is still... mediocre. Patients need to search for each health system, then login to their individual patient portals -- it's pretty high friction. And thats even before you discuss all the barriers that EHRs put up to make it difficult for app developers to register and get production access to EHR systems.
It's certainly possible to integrate directly with EHRs. Most now have pretty good implementations of the interoperability standards, at least for reading data. The problem is that to get access to that data you typically have to establish agreements with individual provider organizations, which is difficult to scale. Many provider organizations also run their own EHR instances rather than using SaaS so for those you also have to deal with all of the endpoint discovery and directory issues.
Working through aggregators such as HIEs eliminates some of those issues.
- [0] https://www.fastenhealth.com/ - [1] https://www.fastenhealth.com/connect/