Because if something is meant to imitate something else, consumers looking for such a substitute product should have an easy means of finding it. The target demographic of these products is people looking to avoid meat, so manufacturers already have an incentive to label them as being meat-free. Making them use meaningless words will inevitably confuse consumers more than a prefix such as ‘plant-based’ would, in turn discouraging adoption of such products by curious consumers, exactly the intended effect of the meat lobby that pushed for inclusion of the provision in the first place.