The world isn't just backend. There are mobile projects, games, native desktop apps which act together as a suite and web frontends. Also microservices bring up their own host of issues and extra labor. Both are essentially the semantic versioning library update problem, and when you need to propagate a breaking change it create a whole bunch more labor that is significantly reduced in a monorepo vs. a multirepo/microservice world.

The tradeoff is you need to invest into repo scaling, much like a backend service that needs to invest in scaling itself too.