I'm not sure I see that horizontally scaling necessarily requires a network call between two hosts. If you have an API gateway service, a user auth service, a projects service, and a search service, then some of them will be lightweight enough that they can reasonably run on the same host together. If you deploy the user auth and projects services together then you can horizontally scale the number of hosts they're deployed on without introducing a network call between them.

This is somewhat common in containerisation where e.g. Kubernetes lets you set up sidecars for logging and so on, but I suspect it could go a lot further. Many microservices aren't doing big fan-out calls and don't require much in the way of hardware.