A couple of things :-

You can still mount filesystems as root from a container, if you have Docker command rights. In Docker's security model access to run docker commands on a given host == root, that's a design choice AFAIK, not an oversight.

It's perfectly possible to mitigate that issue, by restricting who can run containers and also ensuring that all containers specify and use a non-root user account (or enable user namespaces at the Docker daemon level)

Also, many early stage technologies don't prioritise security . For example, for several early releases of Kubernetes all you needed was remote access to a single port (10250/TCP) and you could get root access to the underlying host without any authentication...