The reason that a monorepo facilitates this kind of bit-rot is because of one of the advantages of the monorepo setup: that it also facilitates easily making connections between different parts of the codebase. In a multi-repo setup, you need to be intentional about when and how you pull in another repo, because the bar to doing so is so high. In a monorepo, every piece of every part of the code is available to you and no extra action is required to "pull in" that other repo, which means in practice that without tooling or discipline you end up with a codebase with lots of tight interconnections.

So the reason it happens in a monorepo more is that those connections are easier to make and therefore a lot more common, not because those connections are impossible in a multi-repo setup.