WhatsApp backup on your Google account contains all your message history, and is unencrypted. If you use google cloud backup for your phone, Google can read your WhatsApp messages, and has been able to do so for more than 5 years now.
It is safe to say, optional services get little adoption. Look at email, of the tens of billions of email addresses, there are orders of magnitude lower numbers of pgp or gpg keys. Similar experience with telegram encrypted chats. So having the option of e2ee in a communication platform, is not having it in practice.