The issue with building a custom monorepo system that can handle Google's and Facebook's scale is that it fails to scale down, even to moderately large project and organizational size. It's expensive (think at least 7 figures opex) and not what most people should be doing.

git, for all its issues (and I'm a git-hater), scales down to an individual coder and scales up (with a lot of hacks, the hacks being used varying depending on whether you're taking a poly or mono approach) to companies that employ thousands of developers.