Why do humans not eat meat of carnivorous animals?

On the contrary, we can. Humans are omnivores, which means e eat both plant matter and meat. Salad and herbs are plants and we eat thoses. Most things we eat are plants - except meat obviously.

Hugh Fernley Whittingstall (apologies for an incorrect spelling) is famous for making food dishes whihc contain dandilions, nettles and the likes. Its true our gastric system does have to put a little more effort into digesting plants but we are perfectly capable of eating them.

In fact, more energy is obtained by eating plants than if you eat meats. In a typical food chain, all the energy stored in plants goes to a herbivore, such as a cow, and then we humans eat the cow and obtain energy. However, the original energy is lost as it had to by-pass the cow so to speak. If we simply ate the plant matter, we would gain more energy from our food. No doubt this is a strong thumbs up for vegetarians!