In order to place objects using physics you need to check Simulate Physics Toggle. Please note that a green alert will appear on the top of the Prefab Brush window when physics is running.
When trying to place objects without a physical collider or rigidbody, Prefab Brush will automatically add a Rigidbody and a convex MeshCollider to it. If you want more precision over your collisions, please add the colliders to it manually.
