Agreed about private companies and collecting of data, but in the case of Apple, none of your HealthKit data is transmitted (it's all stored on your iPhone and/or Apple Watch).

Any app that reads/writes HealthKit data is required to provide a privacy policy before it passes App Store approval.

Additionally, apps can only access that data after a full-screen permissions screen is displayed, and the user manually toggles on the categories of Health data that the app can access.

In my experience, the bar for getting apps that use HealthKit is at least slightly higher than for non-HealthKit apps.