RaycastHit2D.collider always null?

I have custom buttons created by sprites, which has component of BoxCollider2D. I want to Raycast for the detection of touch. following is what I set it up:

There are 3 buttons in a Control_3, BtnLeft, BtnCenter and BtnRight, those setting:

And following is the code what I did fot the detection of touch:

 void Update(){
         int i = 0;
         while (i < Input.touchCount) {
             Touch t = Input.GetTouch(i);
             if(t.phase == TouchPhase.Began){
                 RaycastHit2D hit;
                 hit = Physics2D.Raycast(Camera.main.ScreenToWorldPoint(t.position), Vector2.zero);
                 Debug.Log(hit.collider);
                 if(hit.collider != null){ 
                     Debug.Log(hit.collider.name); 
                 }
             }
             i++;
         }
 }

However, the hit.collider always return a null value. I have no idea why would it happen. Could anyone help me, please?