I suspect WhatsApp not encrypting the backups was likely not a technical issue primarily, but a business one. Google gives free storage and can read all your chats on Drive to target advertising. Whatsapp gets free storage.

If you want to protect Google Drive properly, use an "at-rest' encryptor like VeraCrypt[0] or Syncdocs[1]

[0] https://www.veracrypt.fr/en/Home.html

[1] https://www.syncdocs.com/how-to-set-up-google-drive-encrypti...