As mentioned, they don’t have the local capacity to download.
When trying to extract their library to shift it to Google Photos, they hit a number of roadblocks. The full-resolution versions of images and video are thus only held in iCloud. They have a 300GB media library synced with iCloud Photo Library and a modest disk drive on their MacBook Pro, so Photos for macOS is set to optimize media. Macworld reader Shai wrote in with such a concern recently. This often revolves around being able to get a full set of your images and movies in iCloud Photo Library if you don’t have enough storage on your Mac startup volume.
It’s a service that answers many users’ needs, but there are some not-quite-outlying demands that fall through the cracks. There’s no macOS issue I hear about more than iCloud Photo Library.
