Because fault tolerance and isolation has to come with it. It actually has been replicated successfully in modern multiuser operating systems. You can safely r

Because fault tolerance and isolation has to come with it. It actually has been replicated successfully in modern multiuser operating systems. You can safely restart your web server process while the cron service or your DB stays up. One user's shell crash doesn't take down all the other ones. The power of BEAM VM is making it possible to start millions of such processes on a machine. There have been many instances of anyone starting a bunch of threads with some message queues and calling it "a modern super-fast Erlang" but it's just solving the wrong problem.