It's not about migrating APIs or coordinating deployments. That's an impossible problem to solve with your repo. It's to update libraries and shared code uniformly and patch dependencies (eg. for vulns) all in one go.
Imagine updating Guava 1.0 -> 2.0. Either you require each team to do this independently over the course of several months with no coordination, or in a monorepo, one person can update every single project and service with relative ease.
Let's say there's an npm vuln in leftpad 5.0. You can update everything to leftpad 5.0.1 at once and know that everything has been updated. Then you just tell teams to deploy. (Caveat: this doesn't really work as cleanly for a dynamically typed language like javascript, but it's a world wonder in a language like Java.)
I can't fathom how hard it would be to coordinate all of these changes with polyrepos. You'd have to burden every team with a required change and force them to accommodate. Someone not familiar with the problem has to take time out of their day or week to learn the new context. Then search and apply changes. And there's no auditability or guarantee everyone did it. Some isolated or unknown repos somewhere won't ever see the upgrade. But in a monorepo, you're done in a day.
Now, here's a key win: you're really at an advantage when updating "big things". Like getting all apps on gRPC. Or changing the metrics system wholesale. These would be year long projects in a polyrepo world. With monorepos, they're almost an afterthought.
Monorepos are magical at scale. Until you experience one, it's really hard to see how easy this makes life for big scale problems.