But this ignores a lot of the history of Borg. When Borg was first created, it was not nearly as scalable as its current incarnation. We hit scalability bugs and limitations all the time! (I was working on a team which was exploring the scalability limits of MapReduce, which was often very good at finding the limits in Borg and other systems it interacted with.)
Over the years many many Borg engineers have taken on many projects, both in solving bugs and rearchitecting major pieces of Borg with the intention of making it scale better (to run more jobs at once, utilize machines better, increase the degree of failure and performance isolation between jobs, and scale up to manage larger clusters of machines). Many of the lessons learned went into the design of Kubernetes, but Kubernetes is still much newer than Borg, which means it has fewer years of the "identify a scalability bug and squash it" feedback loop.
What is really needed to drive that loop is a major customer pushing the boundaries of scalability and identifying bugs. My guess (from the outside) is that the main users of Kubernetes have been pushing the limits in other directions, which has meant the team has been prioritizing other things (such as improving usability, and adding features) in their development efforts.