Why the browser cannot read your watch over USB

WebUSB only reaches devices whose interface no operating-system driver has already claimed. A Garmin watch enumerates as USB mass storage or as MTP, and Windows, macOS and Linux all bind that interface at plug-in time. The browser is therefore never offered the device. The File System Access API is the supported path: the athlete grants a handle to the mounted folder once, and the page reads the FIT files from it directly.