Because there's so much to properly running it. Individual component updates, network policies, certificates, backups, monitoring, authorization, authentication, secret management. Kubernetes makes it all relatively easy and k3s (or k0s) ain't that scary.
K3s, Calico (or whatever you prefer), Istio with cert-manager and self-manager CA (or whatever you prefer for your service mesh), kube-prometheus, OTEL or Jaeger for the mesh visibility, pick the operator (I used Crunchy Data operator but there are at least two other solid choices), will get one far at low cost. Of course, use reliable infrastructure provider of one's choice.
No need to think about placement (one still can if they want to), addressing, firewalling, DNS, IP assignment, and so on. Add nodes to the cluster as necessary and let it sort itself out.
Some understanding of Kubernetes is necessary, indeed. But it's a stack usable every time once learned.
But why? Patroni is great for HA and it doesn't require k8s.