

Plants are selected to not be great to eat, basically. Cellulose in particular is almost impossible to biochemically break back down (but not completely), and is a pretty good structural material, too.
Seeds are often still palatable once you get through the shell, basically because turning into a baby plant is an already tough design constraint. Some plants still have tricks - notice that it’s the spiciest part of a hot pepper.
In the sense humans are “better” or “greater” or something? Well, consider the global biomass of bacteria compared to humans - they seem to be doing okay. Or that there’s more bacterial cells in you than human cells. Single-celled yeast evolved from mushrooms, barnacles evolved from something like shrimp or crabs, and there are eukaryotes that lost eukaryotic features like mitochondria because they didn’t need them to survive.
Buuut that’s besides the point. I’m not sure how to make it more clear, but I meant subtractive as in selection is just about who dies. Random mutation is what adds features and new species.