The short and pithy answer is because they are idiots.

Denialists love to make claims and link to images of headlines declaring Jewish hostility to the Third Reich. Such newspaper headlines do exist, but much miss much of the context. There were many announced boycotts of Germany and German goods after Hitler's seizure of power in 1933. Not surprisingly, a number of Jewish groups initiated these boycotts, but there were other groups that led these movements. The Third Reich's open antisemitism and violence meted out to Jews in the years leading up to 1933 obviously stimulated a reaction among the Jewish diaspora. None of these boycotts were the result of state policy, nor did they impact American trade policy vis-a-vis Germany. American firms still did business with Germany, albeit under terms increasingly set by Berlin. American-German trade did decline considerably over the 1930s, but private boycotts of Germany were only one part of the picture. Berlin's autarkic drives, shell-games with foreign currency, moves to bilateral trades and protectionism, and German rejigging the German economy for rearmament arguably played a far bigger role in the decline of this transatlantic trade relationship than any boycott. Since these boycotts were not part of the policy of the US government, they were not a hostile act on the part of the United States or any other countries. Calling these acts by private citizens or groups an act of war is as stupid as it is absurd.

While such newspaper headlines did report on the news of the day, relying upon them alone is highly misleading. Much as clickbait today relies upon catchy and extreme headlines, so to did the media landscape of the 1930s. Some Jews in the US and other countries did call for international Jewish solidarity in the face of Nazi antisemitism and dictatorship, but this does not mean "Jews of all the world unite in action." Even a cursory look into Jewish social history will show a strong degree of internal divisions and rivalries.

That such newspaper headlines feature in Denialist tripe is not surprising as one of the staples of Denialism is that even though the Holocaust did not happen, Germany was justified in rounding up Jews into camps because of such declarations of war and hostility. As with much of Holocaust denial, such a conclusion relies upon simplistic thinking and extrapolating questionable conclusions based on very narrow evidence.

And this leads back to the idiocy charge. One of the basic assumptions at work among Denialists is that despite not having any real political power, Jews somehow managed to become the most powerful nonstate actors in world history. The idea that Jews have this massive power would come as a shock to many Jews- past and present- who often bear the brunt of antisemitic violence and discrimination.