AbstractAddStepHandler.recordCapabilitiesAndRequirements(...) has this:

        ModelNode model = resource.getModel();
        for (AttributeDefinition ad : attributes) {
            if (model.hasDefined(ad.getName())) {
                ad.addCapabilityRequirements(context, model.get(ad.getName()));
            }
        }

That 'if' check removes the opportunity for an AD to register a requirement based on the default value of the attribute.

SimpleAttributeDefinition includes logic for doing that, so not giving it a chance to execute is inconsistent. AbstractWriteAttributeHandler also doesn't include any such limitation.