they really are, though. authoritarianism is a sociopolitical organizational concept. fascism is a political ideology. I suggest that you do some reading on the subject.

Fascism is a far-right, authoritarian, ultranationalist political ideology and movement,[1][2][3] characterized by a dictatorial leader, centralized autocracy, militarism, forcible suppression of opposition, belief in a natural social hierarchy, subordination of individual interests for the perceived good of the nation and race, and strong regimentation of society and the economy.

Authoritarianism is a political system characterized by the rejection of political plurality, the use of strong central power to preserve the political status quo, and reductions in the rule of law, separation of powers, and democratic voting.[1][2] Political scientists have created many typologies describing variations of authoritarian forms of government.[2] Authoritarian regimes may be either autocratic or oligarchic and may be based upon the rule of a party or the military.[3][4] States that have a blurred boundary between democracy and authoritarianism have some times been characterized as “hybrid democracies”, “hybrid regimes” or “competitive authoritarian” states.

edit: Soviet Communism was also authoritarian, but it was far-left in its political ideology. no reasonable person would call it fascist, which is far-right.

just because all fascism is authoritarian doesn’t me all authoritarianism is fascism just like how all water is liquid but that doesn’t mean all liquid is water.