Using a monorepo was a deliberate choice at Google. They modified their source control solution to support a monorepo every time it failed to scale sufficiently. If they had started with Git those same people would probably have modified Git to make it work as a monorepo. There are specific economies of scale Google gets with a monorepo that help it keep up with the sheer amount of cruft and technical debt they generate everyday.

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.