Was just wondering because it says do not drink if I drank like half a bottle of listerine mouthwash. Why is that?

Seizures inducer. It's not uncommon to have seizures after drinking Listerine. Thus the bottle suggests it is poison.
Chemicals. The reason you do not ingest anything that is not formulated to be eaten is that it may harm you. The chemicals in listerine are designed to help kill bacteria in your mouth. Swallowing a drop or two accidentally will not hurt you. Ingesting half a bottle will put your digestive system in turmoil and can result in hospitalization. Can't fathom why you would want to make yourself sick.