Comment by gmueckl

Collections and objects have fundamentally different purposes. An object's job is to anchor a bit of geometry information (e.g. a mesh) in the scene. Collections serve as hierarchical grouping and instancing. Instancing is specifically intended to not produce deep copies of objects. This is essential for creating some types of really complex scenes. Any modifier - destructive or otherwise - requires one or more deep copies to act on. Providing modifiers on collections isn't good UX because it blurs the line between what is and isn't an instance.