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.
We eat the seeds, I presume OP refers to the leaves/blades of grass which are also present in species that aren’t cereals
Good point. This made me look further, but some of these also are just the seeds:
https://smartgardenhome.com/edible-plants/grasses/
Bamboo shoots are a common ingredient.
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.