The question is of course, 'What do we and guinea pigs, and only very few other animals have in common. It is of course, the fact that we have to eat vegetable matter to take in the essential vitamin C we need while most animals, particularly carnivores, do not. It is a simple matter to ensure the sickly ones get their supply, for invariably they are having to be syringe fed, see Nursing. I simply put a small portion of a vitamin C Redoxen tablet in the food as I'm mixing it. The need for lots and lots of roughage, in the shape of hay, I put down as the secondary next most important dietary need.
Guinea pigs are herbivores, grazing animals.
Like all animals, their digestive systems have evolved down the ages through the way they have had to forage for their food in the wild. There are no scientifically formulated dry trough feeds out in the wild with lots of lovely protein, minerals and vitamins in them. This is why guinea pigs, like all herbivores, have constantly growing teeth, because of the hard load they have to take on. If they were fitted with the kind we come equipped with they would quickly wear down to their gums. Depriving guinea pigs of the work they have to do to get their food is not doing them a favour and is one of the main reasons for dental problems in the domestic variety.