I've been a big fan of monorepos because it leads to more consistent style and coding across the whole company. It makes the code more transparent so you can see what's going on with the rest of the company, too, which helps reduce code islands and duplicated work. It enables me to build everything from source, which helps catch bugs that would only show up in prod due to version drift. It also means that I can do massive refactorings across the company without breaking anything.

That said, tooling is slowly improving for decentralized repos, so some of these may be doable on git now/soon.