alt Hacker NewsLanguages have affordances. At the extreme ends are PHP, and Java or Go. PHP runs a separate logical process on every request which can't share resources with any other request. Almost everyone using Go is writing a long-running server process because that's how the libraries are designed. Almost everyone using Java is writing a long-running process or a module for one, because Java startup times are obscene. Java also has a very convenient synchronization primitive.
Multi-threaded processes are used because that's the most efficient model, hardware wise. Tasks can maximally share resources. It's not to do with library design or startup time.