Google is the main example, their monorepo setup works and scales really well. It is basically a remote filesystem directory that automatically checks out the files you need to build your project and not other files.
same as pointing to netflix and arguing you need 100+ microservices. no one needs this, maybe except netflix
Google writes a lot of code and ensuring that security fixes get applied or legacy deprecated APIs get removed is for some people a full time job. Having a monorepo makes that job actually achievable rather than impossible.
Source: I worked there and was familiar with many of the engineers working on the tooling for the monorepo.