Why is rigidbody2D calling OnCollisionEnter2D method?

I got 1 gameobject with 1 child under it.

child components:
script with OnCollisionEnter2D(Debug.Log("child"));
circleCollider2D

for parent
boxCollider2D
rigidbody2D
script with OnCollisionEnter2D(Debug.Log("parent"));


Debug Log with all of the above:
parent
child
parent
child
etc.

If i remove parent rigidbody2d:
child
child
child
etc

if i remove parent boxCollider2D:
parent
child
parent
child
etc

Why is it not just the collider calling OnCollisionEnter2D? This seems like the most logical thing that should happen


I noticed if you do the same test but for normal rigidbody and a collider only the rigidbody calls the OnCollisionEnter method Where as if it doesn't have a rigidbody the collider will call it(not both like in the OnCollisionEnter2D case.)
if you do the same experiment with 3d colliders and rigidbodies then if you have a rigidbody ONLY that will call OnCollisionEnter on this gameobject where as if you remove the rigidbody the OnCollisionEnter will happen on the child

Hopefully this is a bug because it is silly that OnCollisionEnter2D and OnCollisionEnter should be called by the rigidbodies (Should be called by the colliders.)

Is this a bug or intended(the 2d part is what i am most interested in).