Why can't I back up my Photos to iCloud?

If you have iCloud Photos turned on (which you do) you cannot also include photos in an iCloud Backup. The yellow portion of the bar graph in your screenshot indicates that the majority of the iCloud storage that you have used is for photos.

Also see the following from What does iCloud back up? - Apple Support

iCloud helps keep your data safe in two main ways: syncing and backing up. For example, if you're using iCloud Photos, iCloud Drive, or Notes, your information automatically syncs to the cloud and is kept up-to-date across all of your devices whenever you make a change. 

Any data on your device that isn't regularly synced is stored in your iCloud Backup. With Backup turned on, iCloud takes periodic snapshots of this data to make sure that it's easy to restore any time you get a new device. 

Between syncing and backing up, all of your data is kept safe in the cloud.

and

Set up and use iCloud Photos - Apple Support

Have you compared the number of photos on your iPhone with the number of photos in the Photos app at iCloud.com and looked at the status of syncing on your iPhone (in other words, why do you thing they are not syncing)? Your screenshot indicates that a large amount of storage is being used by iCloud Photos but there isn't enough information there to indicate there is or isn't a problem.

iCloud is not a backup. If someone got access to one of your devices and maliciously deleted all your photos and then deleted them from the 30 day Recently Deleted folder then they'd be gone for ever. My photos - and all my files - are on TimeMachine backups and plain file backups in my house (mutliple copies), my shed and my parents' house. They are too valuable to trust to someone else. iCloud isn't backup.