Because horizontal scaling is the best scaling method. Moving services to different machines is the easiest way to scale. Of course you can keep them in the same machine until you actually need to scale (may be forever), but it makes sense to make some architectural decisions early, which would not prevent scaling in the future, if the need arises.

Premature optimisation is the root of all evil. But premature pessimisation is not a good thing either. You should keep options open, unless you have a good reason not to do so.

If your IPC involves moving gigabytes of transient data between components, may be it's a good thing to use shared memory. But usually that's not required.

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.