because whatsapp is owned by meta, and their business model is to monetize our data. The agreement between google and meta is not public, but we can be sure it was to monetize our data.
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.