You know all those poisons that kill people? Why doesn't our stomach take care of that?
"Poison" is an extremely broad category. It includes lead, mercury, arsenic, and countless other things. If you put weed killer or insecticide on your mantid's food and it got into the mantid's stomach, the mantid would die, and if you drank weed killer or insecticide it would probably kill you too.
But "venom" is a more specific; it is a chemical that can be a poison, but it is manufactured by a living thing. It is made of protein or some other organic compound and works because it breaks down tissue, or in some cases, interacts with nerve communication. Since venom doesn't kill its host, it must be something that can be easily contained or broken down by whatever tissue holds it.
Venom can be broken down by antibodies, but that is more common in animals that are regularly exposed to it. It can also be broken down (or not absorbed) by a digestive system. Venom is by nature highly toxic in the blood stream and less likely to be toxic in the digestive system.
The only thing that stops it from burining through is the mucous lining on a stomachs.
There is mucous lining in the stomach to prevent the hydrochloric acid from dissolving the tissue, and then a lot of very aggressive digestive enzymes are released in the small intestine to break down the proteins in food. But your body also produces secretions that neutralize those enzymes; bile neutralizes hydrochloric acid, otherwise the acid would rip through the thin intestinal lining.
