The trick is that Google have their own VCS, build tooling, automated refactoring tools, etc etc, specifically designed to deal with their monorepo. Nobody else has that - we're stuck with git and a complex landscape of tools for managing code in ad-hoc ways. As a result, with the tools we have, many repos is better than a monorepo - but perhaps if we had those tools, for some cases, a monorepo might be better than many repos.
Note that even where Google are forced to use git (e.g. Android, Chrome) they use a many-repo approach.
Note that even where Google are forced to use git (e.g. Android, Chrome) they use a many-repo approach.