Note from Garmin site (https://support.garmin.com/en-US/?faq=lK5FPB9iPF5PXFkIpFlFPA):
Data from Connect can transfer to Apple Health. The following is a list of what data types are sent to the Apple HealthKit…
Garmin connect to automatically synchronise and read weight from Apple Health app. This is preventing me from purchasing the Fenix 6 - I’m not spending that kind of money unless this basic function is…
Think about this a bit deeper and understand how much work Garmin would have to put in to read data from the myriad of 3rd parties that permit Garmin to read data from them.
Note from Garmin site (https://support.garmin.com/en-US/?faq=lK5FPB9iPF5PXFkIpFlFPA):
Data from Connect can transfer to Apple Health. The following is a list of what data types are sent to the Apple HealthKit:
NOTE: The Garmin Connect App cannot receive any data from Apple HealthKit. This includes data from an Apple Watch.
It is purely a Garmin issue, and certainly makes their data overview page significantly less useful / sticky.