Because the first one sucked so much, no one wanted to go through it again, so instead of standing up to a bully, they just gave him everything he wanted hoping it would keep him from going to war.
It didn’t.
After the war, Winston Churchill argued it was the most easily avoided war in history. At every stage, it could have been stopped right away with minimal bloodshed, but when push came to shove the British, French and Russians caved when they should have held firm.
In 1936, Hitler sent troops to the Rhineland in clear violation of the Versailles Treaty. It was one division. It had orders to withdraw if the French put up any resistance. The Rhineland is where Germany stationed its troops to start every previous war with France. France protested, but the other nations let it slide. Opportunity one lost.
1938, Hitler threatens to invade Czechoslovakia if the Sudetenland isn’t handed over to him. Czechoslovakia says it’s ready to fight and has border defences and an arms industry to back it up. However, England and France abandon Czechoslovakia and hand the territory to Hitler. Opportunity two lost. Many in the German government were ready to oust Hitler but Hitler had so much support after he succeeded that he was too popular to depose.
1938, Hitler demands unification with Austria. Austria caves. The rest of the world protests but looks on. Opportunity three lost.
1939, Hitler concludes a non-aggression pact with the Soviet Union. Stalin believes he is buying time, but Hitler doesn’t dare invade Poland if the Soviets might retaliate. Instead, they secretly divy it up to return the status of Poland to pre World War I. Opportunity four lost.
1939, Hitler invades Poland. To his surprise, the French and British declare war, but spend the entire fall, winter and early spring sitting behind their defences in France. The Germans have virtually no troops in the west at this time. England and France don’t even bomb Germany. Last opportunity lost.
By May 1940 when the Germans actually attack France, they hit with everything they’ve got. France is still trying to avoid fighting World War I and fights this war the same way - badly. By then, it’s too late. Millions of people are going to die and it’s going to take five years to stop it.