Why the controls are not getting cleared after i pass empty object to the view in MVC?

Question

suppose i have a form like this

@using (Html.BeginForm("submit", "home", FormMethod.Post))
{        
            @Html.TextBoxFor(x => x.firstname, new { placeholder = "firstname" })       

            @Html.TextBoxFor(x => x.lastname, new { placeholder = "lastname" })

    <input type="submit" value="submit" />
}

and i have the method as follows

 public ActionResult Index()
        {                
            return View(new student());
        } 

        [HttpPost]
        public ActionResult submit(student _student) {
            return View("index", new student());
        }

my question is when i make a post request to the same method ie. index by passing an empty object, the controls are getting cleared but when i make a post request to the submit method the value of the controls is not getting cleared. why is that happening? why do i have to clear it using

ModelState.Clear() but not by passing any empty object?