Some background. Germany was in a depression and the government set up after WW1 was weak. Hitler promised stability, the chance to belong to something greater then yourself, pride in themselves and the nation, and finally somebody to hate. He added torchlight parades, fancy uniforms and great speeches.Simply put, the Germans liked him.
Hatred of the Jews wasn't new and wasn't confined to just Germany. Jews were often targets. Hitler first identified them (the had to wear the Star of David) then restricted them, and finally deported them to the concentration camps.
The concentration camps were ill-kept secrets.However, what could be done? The camps were a long ways inside occupied Europe. Russia on the East and the Allies on the West had a hard fight just to defeat the Axis military. By this time the "entire world" had taken sides.
As for "it took so long" It was the preparation, shifting to a war time economy, getting millions of men into uniform, then training them Plans. tanks. ships, belt buckle, rifles all had to be produced, then sent out.
The sheer effort was enormous. The American Army used the M1 Gerand as it's basic rifle. The US produced some 3,526,922 M1 rifles from 1932 through 1945, or 164 rifles per hour.This required building the plants with technology of 80 years ago, getting the materials, training the workers, producing the weapons then shipping them around the world.
More "Gee Whiz" numbers
The liberty ships were essential to transport soldiers and supplies across the Pacific and Atlantic. Eighteen American shipyards built 2,751 of the 14,000 ships between 1941 and 1945. The war for the Atlantic was won partly because America produce ships faster then the Germans could sink them.
With aircraft a partial list was 12,731 B-17s, 19,203 B-24s, 7,366 Lancasters, 13,738 P-40s and 15,875 P-51.
With manpower, some 16 million Americans served during WWII That's out of a population of 132,122,446.
It wasn't that people didn't know or want to stop Hitler, it was the sheer size of the war and how long it took to defeat Germany