Why humans cannot digest grass but can digest lemongrass?

Humans can't digest grass because they don't have those microbes to produce the enzymes that we need to break down cellulose.

Furthermore, the pH of humans’ stomach is normally around 1 to 3, which is too acidic for the cellulose-digesting process to happen, whereas, the pH of the ruminant (cow) is around 7, which is neutral. The microbes stop breaking down cellulose at a pH of 5.5 or lower, so a human can’t digest grass.

Beside the digestion issues, a second problem with grass as a food source for us is that it contains a lot of silica, an eroding agent which quickly destroys the teeth while grazing animal's teeth are well adapted to grow continually and replace the damaged tooth surfaces quickly.

We don't eat lemongrass, rather we commonly used it as a flavouring agent. For example, its leaves are used as a lemon flavouring in herbal teas.

Beside this is used as a fragrance in deodorants, soaps, and cosmetics. Lemongrass is also used in making vitamin A and natural citral.