Apple's health data is all stored offline, unless you have iCloud enabled for health data, in which case it's end-to-end encrypted and only used so devices can

Apple's health data is all stored offline, unless you have iCloud enabled for health data, in which case it's end-to-end encrypted and only used so devices can sync. You can choose to share certain data for their research programs. The answer as to why other companies store data online should be obvious: There's a business can for having access to that data.