I'm inclined to interpret monorepos as an anti-pattern intended to mask away fundamental problems in the way an organization structures it's releases and dependency management.
It all boils down to being an artificial versioning constraint at the expense of autonomy and developer experience.
Huge multinationals don't have a problem in organizing all their projects as independent (and sometimes multiple) source code repositories per project. What's wrong with these small one-bus software shops that fail to do that when they operate at a scale that's orders of magnitude smaller?
That's a huge security problem.
Edit for those who are down voting this post, please explain why you believe that granting anyone in the organization full access to all the projects used across all organizations does not represent a security problem.