Why is the sphere falling through the platform?

I don't want to use physics I'm just testing out something. I have a sphere and you can move left and right with it and I also have a gravity force making it fall down. But when it falls it goes straight through the platform underneath. both the platform and the sphere have colliders. it lands on the platform when i give the sphere a rigidbody but I don't want to use a rigidbody. How do I do it this way so the sphere will land on the platform and why is it falling through the platform?

 using UnityEngine;
 using System.Collections;
 
 public class Player : MonoBehaviour {
 
     Vector2 dir;
     int gravity = 2;
     
     // Update is called once per frame
     void Update () {
     
         if (Input.GetKey(KeyCode.A))
         {
             dir.x = -5;
         }
         else if (Input.GetKey(KeyCode.D))
         {
             dir.x = 5;
         }
         else
             dir.x = 0;
     
             dir.y -= gravity * Time.smoothDeltaTime;
         
         
         transform.Translate(dir * Time.smoothDeltaTime);
     }
 }

EDIT: As I said below Don't let the image put you off. it's just of an image of the sphere at the moment of impact with the platform. It starts out much higher up and falls toward the platform and then falls through.