End to end encryption of messages is by comparison easy as the devices can handle all of that internally. However, losing your iPhone is one of the main reasons to have an iCloud backup. Require a user to come up with a private key and any user who lost it also loses all their data.

Most people don’t really want end to end on consumer backup services, because of the associated risks. If however you don’t want unsecured backups you can handle this manually.

Of course nobody wants the company to actually look at your data, but that’s a separate issue.

Apple doesn't currently have end-to-end encryption for iCloud Photos either: https://support.apple.com/en-us/HT202303