This is absolutely not true, having shared memory or even just being able to communicate over local os pipes is massivley simpler than introducing a network.

And orders of magnitude faster

Sure if you never programmed anything complex that needs actual communication and not just submitting job results at the end

8 servers use up one third of a rack.

You need interprocess communication that works across machines.

You need to orchestrate the deployment of software.

Communication within a machine is much faster.