I'd say that's primarily because those load balancers (assuming some like a Network Load Balancer in AWS for example) are not integrated in the k8s network stack. Cloud load balancers are absolutely an important part of delivering network traffic to your cluster, but the ingress controllers have to "know" a bit more about cluster network topology to serve the purpose they serve. That is, taking incoming traffic from node(s) at a common port and sending it on to a particular service in the cluster.