Generally speaking, fault protection schemes need only account for one fault at a time, unless you’re a really large business, or some other entity with extra-stringent data protection requirements.

RAID protects against drive failure faults. Backups protect against drive failure faults as well, but also things like accidental deletions or overwrites of data.

In order for RAID on backups to make sense, when you already have RAID on your main storage, you’d have to consider drive failures and other data loss to be likely to occur simultaneously. I.E. RAID on your backups only protects you from drive failure occurring WHILE you’re trying to restore a backup. Or maybe more generally, WHILE that backup is in use, say, if you have a legal requirement that you must keep a history of all your data for X years or something (I would argue data like this shouldn’t be classified as backups, though).