But you can make a "new service" in code in the same binary communicating through memory. You can then separate it to a separate binary, when the network and serialisation overhead worth it. 80% of the time it will never come.
Microservices are not only "function calls over TCP" though. There are other concerns such as database per service vs a single shared database. There are also security implications that you don't have with a monolity. Designing it that way from the beginning could easily be as complicated as making them separate services.