I'd agree that k8s has a lot of functionality built-in, another important thing to realise is what k8s doesn't do.

In addition to the well-known integration points (Container Runtime/Network/Storage Interfaces), there's things like the lack of a good built-in user authentication mechanism with Kubernetes, which means you pretty much always need some external authentication service for your clusters.

That's not too bad if your on one of the big managed providers (GKE/AKS/EKS) but can get complex for people who want to deploy on-prem.