Defending the cluster from malicious nodes is not in the primary threat model of Kubernetes today. A malicious node can do many things, like lie about its capacity, scan container memory for juicy secrets, inject arbitrary outgoing or ongoing traffic, and in general be a jerk.

Securing nodes, preventing container escape, subdividing role access, constraining placement, limiting master surface area, and end to end audits have been the initial focus. Until those are in place, node secret access was less critical.

It is something that several folks are interested in working on soon.