It is because Google wants a monorepo, then Google choose to use Perforce (and later Piper). It is not that Google uses Perforce and thus are limited to a monorepo.
The core value behind monorepo (and monorepo-like approaches) explained in the book is that dependency management is harder than version control.
The core value behind monorepo (and monorepo-like approaches) explained in the book is that dependency management is harder than version control.