When checking your nodes resource consumption, I can see that you have requested much more CPU than you actually use. None of your nodes has less that 60% of requested CPU so the cluster autoscaler can’t drop any node.
But indeed the utilization is very low. I encourage you to review the resources assigned to the services in the resource settings to ensure they are aligned with actual usage.
In this post, you can find a complete explanation of how Kubernetes resource allocation works
I hope it will help
Cheers