Group in Group (shapes Layers)

Hello,

Why can't create a group in group ?

var myComp = app.project.activeItem;
var shapeLayer = myComp.layers.addShape();
var contents = shapeLayer.property("ADBE Root Vectors Group");
var group =contents.addProperty("ADBE Vector Group");
var littlegroup =global.addProperty("ADBE Vector Group"); 

thanks