For a complex project you need complex tooling. A mono-repo and a multi-repo system have different needs, but both need complex tooling to work. Neither is inherently better than the other, there are pros and cons. Sometimes those are compelling (which is why a few projects at google are not part of the mono-repo)

For me I prefer the pros and cons of multi-repo. However sometimes I wish I could do the large cross project refactoring that a mono-repo would make easy.