Because shit can happen even when you're backing up...?

what kind of shit can happen without you knowing when backing up?

Power loss. I mean, backing up is the literal definition of a nightly job...

So, what does a journaling FS helps in a power loss? You just restart backup anyway. What’s the concern there?

The health of the backup device. Restarting a backup is one thing, having to rebuild a disk is another.

You don't need to rebuild a disk because some clusters were orphaned. It doesn't mean disk was corrupt, it just means there is some allocated space that isn't used. It's trivial to fix too. There is no danger there. Actually, what journaling does is to rollback that allocated space. Doing that manually doesn't make it less safe. For backup scenarios exFAT is perfectly feasible.

What is feasible and what is desirable are two different things.

With journalling, I don't have to know or care about any of this: I restart and chances are it's all back to normal. This is desirable.

You can store your backups on stone tablets, with a machine that carves rock to write 1s and 0s and a conveyor belt that feeds new tablets. That is perfectly feasible. It is also not desirable.