There’s a reason grass is so common - it’s because it’s a wildly effective life strategy. Grass is actually quite hard to eat - there’s basically no nutrition in the leaves themselves, and grass evolved to incorporate silica “needles” in its leaves, so that it wears down your teeth when you try to eat it anyways.

Not to say that it’s impossible to eat grass, but you need to undergo a ton of highly specialized adaptations to make it possible. For most animals (including humans), it’s just not worth the effort

Smarts required more calories.

All forms of grass:

• Corn
• Wheat
• Barley
• Rice
• etc

Grass is mostly cellulose and lignin. Those molecules are difficult to break down.

Animals that can digest the cellulose either need a really long digestive track or to do something really gross to keep the stuff in their digestive track longer.

A lot of grass. Like lawn grass. Is wheat.

If you let it grow more.

So actually we did evolve to eat grass.

Cat grass is wheat grass.

The thing is that cows can’t digest grass either. They have an extra stomach along their oesophagus which is basically just a pouch where the grass goes in first. There are a lot of bacteria and they can digest grass. Then these bacteria grow because they eat the grass.

Then the cow swallows these bacteria and digests those. That’s where a cow gets their calories from.

It’s all down to the way the brain works. Our brains use up something like 20% of our calories when standing still doing nothing.

Grass does not supply the amount of calories and micronutrients needed to keep the human brain running, simply because it is low on both of those things.

Grass eaters have multiple stomachs, slow digestion and graze pretty much the whole time they’re awake, and because their brains use a lot less energy than human brains, the balance works out.

It’s more efficient to let others eat the grass, and then eat them

Take the tier zoo aproach. Would you rather use evelution points on grass or evolution points on being big brain.

Vegetables have pretty limited availability for protein, as an animal you have sit there eating grass all day. Our ways allow us plenty of time to be smart & stuff

Return to cow

I’d rather evolve the ability to photosynthesize, now that we have indoor lighting. It’d save a whole bunch of time and grocery bills.

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

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.

That random mutation didn’t happen, basically.

Evolution is a purely subtractive process. It doesn’t design things in, it just removes poorly-designed creatures (and all hypothetical offspring) until only things equipped to survive are left. And obviously, there are things to eat that aren’t grass.

Edit: Herbivores can be smart, even the grazers. Look at elephants.

I can’t believe how many other replies heap that fallacy on top of teleological evolution. Apes are mostly herbivorous anyway, WTF.