Because it requires a lot of biological investment to eat it. It’s rough on teeth and requires rumination or similar calorically expensive techniques to extract much nutrition. We evolved in the opposite path and optimized heavily for easily digested foods. We then take it a step further and cook them breaking the difficult to digest parts into an easer to digest form.

Also we do eat grasses, but only their seeds and fruits. Wheat, maize, rice, and bananas are all grasses