I'm a huge critic of git, but I think it won out on merits. Git was used by the Linux kernel, and therefore from the beginning scaled better to much larger projects than other DVCS systems.
I don't really see a benefit of having all code of an org in a single repo. Single product, sure. But whole company? Why. Not to mention, these are serious outliers.
From security standpoint, it doesn't seem great, from practical side, it's not great (bandwidth cost ect).
Actually, it scales well with large codebases, but the monorepo mental model pushed by FB/Google doesn't match the git workflow model. Git repo design is for a concrete entity: a single tool, binary, service, kernel, whatever. When you start bashing in multiple unrelated projects where you try to have various per-project versioning schemas, it becomes a mess.