Because the American far-right is even further to the right and more insular than the Liberals. Just because both are politically right-wing ideologies doesn't mean they are friendly with each other. Same thing happened in Nazi Germany, the fascists hated the liberals then too but the liberals still played the concession game thinking they could keep the monster tamed.

Then there is the decades of political manipulation through the shifting of the Overton Window and the Liberal establishment co-opting leftist movements by claiming they are "the left" in order to hamstring any discussion from challenging capitalist interests making things even more confusing for those who do not have enough understanding of political science to know the differences.