Because mutable persistence in Kubernetes can be super annoying to manage and people might grasp for whatever lifeline they can find.
If you have a managed object store or even relational database outside of k8s, the thought of storing arbitrary data in secrets probably doesn't come to mind. But if your enterprise spools up a cluster and tells you to use nfs PVCs with no other storage solution, suddenly you might start getting creative.
What makes you think Kubernetes "secrets" are appropriate for storing secret information? They're not secure (not without adding a bunch of other nonsense on top of them).
— Douglas Adams