It's included in iCloud backups, just not toggled to use end to end encrypted iCloud sync (in which case iCloud backups would back up the decryption key).

See https://support.apple.com/en-us/HT207428, https://support.apple.com/en-us/HT208532, https://support.apple.com/guide/security/security-of-icloud-..., and https://support.apple.com/en-us/HT202303