Swap is off because turning it off is ultimately good for performance. Kubernetes isn't designed around the philosophy of using swap, instead it uses memory limits, and if there is not enough memory on a node then it will schedule new pods on a different node. As a result, the Kubernetes team hasn't built in support for using swap since they don't think you should be using it. In the end, kubelet won't work if swap is on.
With swap off, it will definitely be bad if you run out of memory. However, Kubernetes is able to prevent that from happening. It limits the available memory to pods, and won't schedule pods that need more memory than is available on the system. Technically, they could modify Kubernetes to support swapping, but the Kube team decided it wasn't worth it due to higher priorities, and because they want to instead stick to the philosophy of not using swap. Basically, they didn't want to spend a bunch of time supporting something they thought most people shouldn't be doing anyway. Basically, they see it like this: Kubernetes is a tool that gives you the ability to manage your resources effectively enough that you don't need swap, so they don't support it.