MyChart still not in Health app?

You connect to your specific health provider in the Health app. Here is the list of supported providers.

https://institutions.healthrecords.apple.com

Once you choose your provider, you may be asked for your MyChart login if that is one of the options. To add your health records, including medications, immunizations, and lab results, follow the information provided here:

View health records on your iPhone or iPad - Apple Support

Apple cannot get your data from an organization that is not wiling to share it. As you can see from the list it is quite extensive and see know reason why Apple would refuse to cooperate. One thing to check is if your health care provider has a parent company that is listed. Many individual providers are managed by much larger health care networks. For example, in my area there is CHI Franciscan that is used for literally hundreds of physicians, clinics, and hospitals in my area.