Asklemmy

Essentially as others said, because you need to invest heavily in your gut and metabolism to get enough energy out of grass alone. You don't evolve into what you want, you evolve into what you can while you are pressured to do so. There is currently no pressure to rely solely on grass, that pressure hasn't been on us for millions of years. Our foods may be trickier to find but on average they yield more energy compared to grass.

But don't get me wrong. It's a valid strategy. Our ancestors did have the window of opportunity at some point, and they took it... We were something more like rats back then, the grass eaters niche had room. Grass was everywhere. And our cousins back then adapted and spent practically the whole day foraging for grass in order to get by. But you know what also was everywhere? Trees. And our ratlike ancestors that were more inclined to climb, jump, and spot predators from above found it easier to stick to the omnivore diet.

Adapting to trees lets you move into places with less grass... Like jungles and swamps. And while you are there for a couple million years you will have other problems to solve, problems that require wits, sharp vision, and social skills. You don't have time to forage 18hrs every day and grass isn't as abundant here anyway.

If you don't rely solely on grass you will probably fare better during winter. Especially during those ice ages. By the time you have grass as an option again you are pretty much an ape and pivoting back to it makes no sense.