But imagine the increased productivity of your devs if they only had to check out a single repo. Anyone has the same organization of projects on their machine. All tools are in one place...
A: You avoid issues such as Readme files stating, "before compiling you have to git clone ../commonA, ../commonB". These always tend to get stale so in reality you also have to git clone ../commonC wasting you tons of hours of troubleshooting.
B: Developer working on daily basis in component A finds a bug in component B. He just has to change the code and commit it for review, instead of understanding the specifics of working with component B repository.