Hindsight is 20/20, but during the actual holocaust people did not always know what was going on, and they either couldn't, or didn't want to, fathom as bitter of an end as we know it today. In 1939-42 Europe it was inconceivable that by war's end the continent would have witnessed the cold blooded murder of 6,000,000 Jews and the destruction of hundreds of millennia-old Jewish communities.

The Nazis were not only superior fighters, they were also great propagandist, and went out of their way to deceive the Jews¹. As the Jews were forced to board cramped cattle cars they were lead to believe they were being relocated to better living conditions. Thousands of Jews undressed and entered the gas chambers believing they were entering showers or disinfectant rooms.

The secret couldn't be kept forever and in the later years word of the genocide started getting around. "Rumors" began to spread that the trains were actually "relocating" people to their death, and that the work in "work camps" consisted of exterminating people and disposing of their bodies. 

At this point armed and organized resistance didn't seem plausible. The remaining Jews were weak, dispersed, confined, unarmed and facing the experienced Nazi war-machine. They feared any attempt at organized resistance would be ineffective, and worse, it would only bring a stronger backlash, enraging the Nazis and causing them to kill, torture and maim even more Jews. (Which is what always happened after failed uprising attempts).

There were, nonetheless, several attempts at armed/organized resistance, most famous of which is the Warsaw Ghetto uprising, where the Jews put up a good fight with little arms, ammunition or man-power, and withstood the full force of the German Whermacht war machine for over 30 days. Ultimately the Nazis leveled the Ghetto.

Another most daring uprising took place in the Sobibor death camp. The uprising is recorded as a successful uprising, although only 300 of the 600 prisoners made it out of the camp alive. The other 300 were killed during the attempt. Of the 300 that made it out, only 50 survived the next two years of the war. Most were killed within a few days of the uprising by the Nazi forces or Polish villagers.