Why does the recusrvie part of .extend limit itself to plain Obect?

I don't understand why the extend method doesn't recursivelly (when deep = true) extends properties of custom objects type.

function CustomObject() {
}

var myCustomObject = new CustomObject();

var obj = { element: myCustomObjec);
var copy = $.extends(true, {}, obj);

obj == copy // false
obj.element == copy.element // true