Why does my onBecameInvisible never called?

I have 3 rocks as prefabs on Unity, I attached this code to the camera and added them as the Rock Prefabs.
The idea is that I create 3 rocks, and wait until they are out of the screen, and every time I destroy 1, create a new one. The thing is that the onBecameInvisible method is never accessed. What am I doing wrong?

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class createRock : MonoBehaviour
{
// Object
[SerializeField]
GameObject rockPrefab;

// spawn control
const float MinSpawnDelay = 1;
const float MaxSpawnDelay = 2;
Timer spawnTimer;
float allowedRocks = 2;

private GameObject[] numberOfRocks;


// spawn location support
const int SpawnBorderSize = 100;
int minSpawnX;
int maxSpawnX;
int minSpawnY;
int maxSpawnY;

/// <summary>
/// Use this for initialization
/// </summary>
void Start()
{
// save spawn boundaries for efficiency
minSpawnX = SpawnBorderSize;
maxSpawnX = Screen.width - SpawnBorderSize;
minSpawnY = SpawnBorderSize;
maxSpawnY = Screen.height - SpawnBorderSize;

//Check the amount of rocks


// create and start timer
spawnTimer = gameObject.AddComponent<Timer>();
spawnTimer.Duration = Random.Range(MinSpawnDelay, MaxSpawnDelay);
spawnTimer.Run();
}

/// <summary>
/// Update is called once per frame
/// </summary>
void Update()
{
// check for time to spawn a new rock

numberOfRocks = GameObject.FindGameObjectsWithTag("rockCreated");

if (numberOfRocks.Length <= allowedRocks && spawnTimer.Finished)
{

//print("The Number of rocks is " + numberOfRocks.Length);

createAnotherRock();

// change spawn timer duration and restart
spawnTimer.Duration = Random.Range(MinSpawnDelay, MaxSpawnDelay);
spawnTimer.Run();
}

}

/// Spawns a new rock at a random location
void createAnotherRock()
{
// generate random location and create new rock
Vector3 location = new Vector3(Random.Range(minSpawnX, maxSpawnX),
Random.Range(minSpawnY, maxSpawnY),
-Camera.main.transform.position.z);
Vector3 worldLocation = Camera.main.ScreenToWorldPoint(location);
GameObject rock = Instantiate(rockPrefab) as GameObject;

rock.transform.position = worldLocation;
}


void OnBecameInvisible()
{

Destroy(this.gameObject);
}

}

 

You didn't use code tags:
https://forum.unity.com/threads/using-code-tags-properly.143875/

And you didn't put the OnBecameInvisible on the rocks which you wish to destroy upon invisibility.
https://docs.unity3d.com/ScriptReference/MonoBehaviour.OnBecameInvisible.html

Also make sure the scene camera doesn't see them either. It counts.

 

I actually tagged the rocks manually, that is why I could use numberOfRocks.Length to stop creating rocks after 3.
Would that work? Any other idea?