It's certainly not the case that no open source projects use monorepos, you are right that the BSDs do as well.

It's that those repositories are still orders of magnitude smaller than the ones that are inside of companies. Monorepos tend to be larger than polyrepos, due to the fact that you've got everything all in one repo, but that doesn't mean all monorepos must be truly large.

The real issue with the open vs closed bit is that features like "this subdirectory of the repository is only writable (or even visible!) to some people" is basically nonsense for open source, but valuable to a company monorepo. Their needs are just different.