Pretty sure even if Google doesn't support it explicitly, you'll have open source tools that can download + re-upload to Google.

That assumes Google provides API and authorizes the access, or else you're stuck scraping complex web apps with a Chrome extension.

Custom 3rd party software is usually banned from a consumer cloud API for security reasons.

I'd say it's pretty common with cloud providers actually. Bring in data? (Spotify playlists, Photos, Contacts etc. etc.) come on in, we have great APIs. Want to take your data out? File a GDPR request and get it in some opaque format.

No incentive for Google to explicitly refuse photos from Apple for example.