That's the myth that was started by the Progressives after Hitler invaded Poland and reports of the atrocities the USSR perpetrated on their own people came out.
Prior to that the Progressive movement in America were praising Hitler, Stalin and Mussolini as "men of the future". The Progressive movement went underground and most left when they saw what Marxism really was. But the die hards just kept their mouths shut until the 50's and 60's.
Because we went to war with Germany and Italy, the Progressive who had already claimed a strong foothold in our education system, started teaching that the Nazis and Fascist are on the far right. They came up with all sorts of bizarre justifications as to why and really, really hoped that no one would actually take a serious look at Nazism and fascism and compare them to socialism and communism.
Unfortunately for them, there are people who still question things. There are people who remember the rallies the Progressives once had where they all gave their devotion to Marx, Hitler, Stalin and Mussolini.
Sorry but Nazism and fascism were born out of socialism just like communism. What they teach in schools is a lie. They all have been strongly influenced by Marx. They all believe the government should control the means of production. They all believe the rich must share their wealth with everyone else, they all promised free healthcare, free education, pretty much free everything. They all believed in the concept of social justice, where the oppressors must be made to pay for their crimes.
They are all forms of authorization collectivism with minor differences to the people on the outside but to those on the inside they are massive. That's why the Italian socialist murdered Mussolini, he wasn't doing socialism the right way. It's why Hitler murdered the German socialists, he didn't want them criticizing his form of National Socialism.
There is so little difference between any of them that there is no room for freedom in the middle. Fascism may be on the right but it is just a hair's width to the right of socialism.
The true extreme right is anarchy, no government at all. Total freedom for everyone to do anything, even if it violates the rights of others.
Our founding fathers put us just to the left of anarchy but miles to the right of authoritarianism.
This myth has been spread for so long that it has built upon itself multiple layers and the majority of Americans have just come to accept it as truth.