There are many reasons not to do it. One example is the compartmentalization of confidential, sensitive, or secret business logic and programs.

nope, that's completely incorrect, google does "compartmentalization of confidential, sensitive, or secret business logic and programs" in the monorepo. it's also massively increased how often it does and expanded how it works as management trusts employees less and less and less over time.

You missed the part where I said with good tooling. Nothing about a monorepo requires you to have all of the repo accessible to everyone.