Because dogma. There are tons of places running production postgres, and indeed many other stateful services, in Kubernetes.

Edit because presumably GP downvoted me for contradicting them, since I've personally overseen this in production at Fortune 100 companies and unicorn startups alike:

https://dok.community/

https://github.com/zalando/postgres-operator

And plenty of YouTube videos from various kubecons and CloudNativeCons. Kubernetes is a runtime and provides plenty of primitives for safely running stateful workloads even better than otherwise possible. Anyone who says otherwise hasn't bothered directly learning enough about the possibilities and is likely citing oft-quoted dogma that dates back to the earliest days of k8s and was questionable even then.