Well, there are encrypted CNIs like Weave. I've used Calico over ZeroTier to similar effect. The network is 'encrypted' and there isn't much effort required past initial configuration.
But that's not really the issue. You still have a big plaintext network with a bunch of random stuff talking, no mutual auth and no security controls other than segmentation. That's the tricky problem that mTLS and service meshes attempt to solve.
But that's not really the issue. You still have a big plaintext network with a bunch of random stuff talking, no mutual auth and no security controls other than segmentation. That's the tricky problem that mTLS and service meshes attempt to solve.