Why do I get null instead of empty string when receiving POST request in from Razor View?

I used to receive empty string when there was no value:

[HttpPost]
public ActionResult Add(string text)
{
    // text is "" when there's no value provided by user
}

But now I'm passing a model

[HttpPost]
public ActionResult Add(SomeModel Model)
{
    // model.Text is null when there's no value provided by user
}

So I have to use the ?? "" operator.

Why is this happening?