Many herbivores have a part of the digestive tract devoted to fermentation (or other microbe based processes) to break down cellulose. This involves a community of microorganisms that live in that part of the gut, and it is those microorganisms that break down the plant matter, producing nutrition for the animal via the products of their digestion, or by the animal breaking down the microorganisms themselves. Ruminants in particular like cows with their specialized multi-compartment stomach devote a lot of space to culturing this microbe colony, but rabbits and horses are hind gut fermenters so they have cecum for that. Rabbits also are coprophagic (eat poop), they digest some of their plant matter once, then eat the poop pellet and send it through again so it can be broken down even more.
But basically, with the microbes doing the work of digestion, it is more about what they can extract, and the herbivores just host them. We have a different community of microorganisms than them, and our digestive tract wouldn’t be able to support large numbers of those species.
Fair criticism, and in regards to minerals especially, I totally failed to mention the need for herbivores to have access to literal rocks and dirt rich in different minerals that aren’t readily available in plant. In captivity, this takes the form of mineral blocks of course.