You missed the point. It is not about if it is private or not, it is how they use it.
You are allowed (on some pages) to read news article. Are you allowed to copy and publish them on your own site? No.
You have a Copyright on your posts same as a author has on his books.
If it is legal or not is still to be discussed.
Similar to how data was mined (or even still is) about users without consent. Now there is for example the GDPR.
You can just backup the Whatsapp directory yourself. Whatsapp creates a message backup file every day or two, even when Google Drive is disabled.
To recover them you need to copy them over. Not well documented.
There are even reported cases where Microsoft support used that tool to activate Windows Licenses when there are problems with the License of a customer.
Not quite sure why, they simply could in the fly stitch those files together.
Twitch is doing that for a while now i think.