Because they do nothing to make it hard to add a coupling or break modularity.
You should of course use good discipline to ensure that doesn't happen. Compared to mutli-repo it is a lot easier to violate coupling and modularity and not be detected. Anyone who is using a monorepo needs to be aware of this downside and deal with it. There are other downsides of multi-repo, and those dealing with them need to be aware of those and mitigate them. There is no perfect answer, just compromises.
You should of course use good discipline to ensure that doesn't happen. Compared to mutli-repo it is a lot easier to violate coupling and modularity and not be detected. Anyone who is using a monorepo needs to be aware of this downside and deal with it. There are other downsides of multi-repo, and those dealing with them need to be aware of those and mitigate them. There is no perfect answer, just compromises.