Good question, I'm also interested in the answer. Probably has something to do with K8s being backed by Google, which has battle tested it in the form of their internal Borg infrastructure.
My understanding is that k8s was definitely never used internally at google (at least at the beginning). They may have learned some lessons from Borg, but k8s is very very much not Borg nor a Borg component. I remember lack of battle testing and architectural differences being a key criticism when k8s was released.
