Different organisms have different metabolic pathways. Humans are unable to synthesize 9 of the 21 most common amino acids, and must acquire them from food. To learn more, look up "Essential Amino Acid".

Cows do it by having a complicated multiple-chambered stomach full of specialized bacteria that can digest cellulose -- basically a natural chemical engineering facility.

Humans don't have that.

Maybe we should genetically engineer humans to be more like cows, instead of genetically engineering mold to be more like meat. ;)

Why would you assume our bodies work like cow bodies? We are very different animals.