This is a fair enough point, but you should also try to keep that tree as small as possible. You should have a damn good reason to make a new service, or break an existing one in two.

People treat the edges on the graph like they're free. Like managing all those external interfaces between services is trivial. It absolutely is not. Each one of those connections represents a contract between services that has be maintained, and that's orders of magnitude more effort then passing data internally.

You have to pull in some kind of new dependency to pass messages between them. Each service's interface had to be documented somewhere. If the interface starts to get complicated you'll probably want a way to generate code to handle serialization/deserialization (which also adds overhead).

In addition to share code, instead of just having a local module (or whatever your language uses) you now have to manage a new package. It either had to be built and published to some repo somewhere, it has to be a git submodule, or you just end up copying and pasting the code everywhere.

Even if it's well architected, each new services adds a significant amount of development overhead.