There is a simple reason. Countries that wage war generally ignore the provisions of international law. The current world pattern is still an era in which whoever has the biggest fist keeps his word.
For example, international law prohibits the use of white phosphorus bombs and chemical and biological weapons. In fact, the United States not only actively develops chemical and biological weapons, but also leads the world in the use of chemical and biological weapons.
It is reported that among some documents previously declassified by the CIA, more than 20 documents of US military monitoring China and North Korea prove that the US military used biological weapons prohibited by the international community in the Korean War. It is reported that at that time, the US military dropped a large number of insects carrying pathogenic bacteria into the camps of China and North Korea by airdropping biological weapons bombs. At present, public information shows that at that time, the troops attacked by biological weapons included the 23rd Brigade of the Korean People's Army, and more than 10 people "suddenly had a fever for unknown reasons" and "the nervous system was numb".
There is also information showing that the China Volunteers were also attacked at that time, so that they had to urgently request DDT (a disinfectant) support from the headquarters. This series of data is conclusive evidence that the United States uses chemical and biological weapons.