As a gut check, most of these reasons apply to AWS AZs as well. If your Kubernetes strategy calls for more than one per region per AWS account, it means that you, organizationally, don't trust containers as much as you trust VMs on AWS. And right now, you're probably right to do so.
Treating a single cloud account and the clusters it hosts as a single security cell is more than reasonable, especially if you need to handle sensitive data, be able to audit the state of the cluster (hope your cluster can’t overwrite the contents of the S3 bucket that you’re using for cloudtrail logs, etc), or reason about the risk of compromise.
We run lots of multitenant kube clusters, and I would still recommend anyone who expects to grow to create hard walls between clusters and cloud infra as soon as reasonably possible.
Multiple Kubernetes clusters for workload separation and AWS accounts for workload separation aren't quiet the same and bring different levels of complexity depending on what your internal processes look like.