Add custom event on child process

In main file I have this code:

var forest = require('child_process').fork("forest.js");
    forest.emit("go",12,15);

In forest.js:

process.on('message', function(message) {
    console.log("Normal message: "+ message);
});
process.on('go',function(x,y){
   console.log("EVENT GO THERE:"+ x +"/"+ y);
});

Why when I emits "go" event nothing happen?

I find similar question, and answer which resolve issue: http://stackoverflow.com/a/16509846/2490611. Why I need do this in that way, and can't add listener directly to process ?