We need to track large external projects (buildroot, the Linux kernel for instance) so the ability to include them as submodules and update them fairly easily is worth it IMO. If you're at the scale of Google it probably makes vastly more sense just including the code in your monorepo and pay a bunch of engineers to merge back and forth with upstream and have the rest of your team not worry about it, but for us it would take a lot of time and effort to maintain a clone of these projects in a bespoke repository.