From viewpoint of security and separation of concerns giving unlimited access to everything by virtue of "everything" being stored in one giant repo sounds exceptionally short-sighted. A single rogue actor would be able to insert code to any component of choice instead of working on isolated repo with people who specifically know it and approve the code: the monorepo is a "big ball of mud" with vague shared responsibility that defers to people who worked on "specific parts" but they lack any authority or control, auditing the entire codebase doesn't scale.