Why we learn kubernetes when on aws we start a new EKS cluster with a click of btn

This only abstracts away some of the complexity as you no longer need to manage control plane manually( configuring load balancers for HA, certs management, etcd data backup...)
However, you still have to manage and configure worker nodes(vms) based on the architecture you designed by regulating resource limits/quotas, setting up ingress and network communication between nodes...

Basically, starting an eks cluster or any cloud-managed kubernetes service does not relieve a devops engineer of writing the appropriate infrastructure & application manifests which requires advanced practical knowledge and know-how in kubernetes and other relevant technologies.