Collider bounds encapsulate doesn't seem to do anything

Hi, today i ran into the following problem: I need to add a (Box)Collider to a GameObject loaded from an AssetBundle. Since it is a complex geometry, i don't want to add colliders to all children of the root node, but rather just add a collider to the root node and grow it to the size of the full geometry.

From what i read so far all i would have to do is call collider.bounds.encapsulate on all the children of the root node. However when i do that, nothing at all changes about the collider, no matter what i try.

In the code below i made a simplified example of what i mean (instead of loading a geometry i just create 2 primitives) and in the comments you can see the Debug.Log result.

As you can see no matter what i try the bounds of the default boxCollider of my cube primitive stay the same.

So my question is what am i doing wrong??

 public class test : MonoBehaviour
 {
 
     void Start()
     {
         var cube = GameObject.CreatePrimitive(PrimitiveType.Cube);
         cube.transform.position = new Vector3(0, 0, 0);
 
         var sphere = GameObject.CreatePrimitive(PrimitiveType.Sphere);
         Destroy(sphere.GetComponent<SphereCollider>());
         //I intentionally remove the collider, since i want to grow 
         //the cubes collider to include the sphere
         sphere.transform.SetParent(cube.transform);
         sphere.transform.localPosition = new Vector3(2, 0, 0);
 
         cube.GetComponent<BoxCollider>()
             .bounds
             .Encapsulate(sphere.GetComponent<Renderer>().bounds);
 
         Debug.Log(cube.GetComponent<BoxCollider>().bounds); 
         //Result: Center: (0.0, 0.0, 0.0), Extents: (0.5, 0.5, 0.5)
 
         cube.GetComponent<BoxCollider>()
             .bounds
             .Encapsulate(new Bounds(Vector3.zero, new Vector3(100, 100, 100)));
 
         Debug.Log(cube.GetComponent<BoxCollider>().bounds); 
         //Result: Center: (0.0, 0.0, 0.0), Extents: (0.5, 0.5, 0.5)
 
         cube.GetComponent<BoxCollider>().bounds.Expand(500);
 
         Debug.Log(cube.GetComponent<BoxCollider>().bounds); 
         //Result: Center: (0.0, 0.0, 0.0), Extents: (0.5, 0.5, 0.5)
     }
 }