You have one commit hash in one repo that tells you what version service/consumer X is expecting / providing.
If you want to understand why a service isn't working as expected, it's trivial to grep its implementation and contribute solutions.
As a service owner, you can grep for all places where your service's client is initialized, and update them in one PR (vs 12 PRs in 12 repos, that you have to manage independently).
Basically, it reduces the coordination cost of breaking changes.
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You can get these benefits in a multi-repo setup only if you have adequate tooling around multi-repo PRs, code search, etc. It's not impossible, but the barrier to highly effective work is higher, imo.